Light

A ray of light that strikes a surface or falls on a surface before being reflected, transmitted or absorbed is known as incident ray. For eg-

The light ray reflected by a surface, when an incident ray strikes the same surface is the reflected ray. The reflected ray corresponds to the incident ray always.

For a single incident ray falling on a plane mirror, there is always a single reflected ray. The angle of incidence is always equal to the angle of reflection, this is by the law of reflection.

The point of incidence is the point where the incident ray strikes the mirror.

Normal is an imaginary line perpendicular to the surface, in the reflection of light from a plane mirror.

The angle which incident ray makes with the perpendicular to the surface at the point of incidence.

The angle between the reflected ray and the line perpendicular to the reflecting surface at the point of incidence is known as the angle of reflection.

The angle of reflection will be 30 degree. This is by the law of reflection that angle of incidence is equal to the angle of reflection.

The angle made by the normal will be 90-75= 15. Angle made by normal is equal to angle of incidence. So i=15 degree. And by the law of reflection which states that angle of incidence is equal to the angle of reflection, the angle of reflection will be 15 degree.

When a ray of light is incident normally (perpendicularly) on a plane mirror, the ray reflects and follows the same path.

The angle of incidence when a ray of light is incident normally on a plane mirror is zero degree because the ray is incident normally .

The angle of reflection when a ray of light is incident normally on a plane mirror will be zero degree because the angle of incidence will be zero degree as the ray is incident normally. By law of reflection, angle of incidence= angle of reflection.

the angle of incidence= angle of reflection.

According to the question, given i+r= 90 degree

i=r

hence i+i=90 degree

2i=90 degree.

I=45 degree.
Therefore, the angle of incidence is 45 degree.

The apparatus used to obtain a thin beam of light is known as Ray box. The rays are bright enough to be viewed easily. A ray box is a standard source of light used in optics experiments.

a) Diffused reflection takes place from a rough surface. The reflected rays are not parallel in diffused reflection.

b) Regular reflection takes place from a smooth surface. The reflected rays are parallel in regular reflection.

Regular reflection, wherein the reflected rays are parallel to each other, lead to the formation of images because all the rays get reflected in a particular direction. Regular reflection always occurs from a smooth surface.
Diffused reflection does not lead to the formation of images because the rays do not reflect in one particular direction.

a) Regular reflection occurs from a cinema screen because the surface of the screen is smooth.

b) Regular reflection occurs from a plane mirror because the surface of a plane mirror is smoot/ even.

The object would be 15cm far away from its image if it is placed at a distance of 7.5cm from the plane mirror.

The object is at 7.5cm from the mirror, then the image of the object is 7.5cm on the other side of the mirror. Hence, the image is a total of 15cm from object (7.5cm to the mirror+ 7.5cm to the image).

The image of an object in a plane mirror is virtual. Virtual image cannot be projected onto the screen because it is not real.

A virtual image is a copy of the object, formed on the plane mirror. The light rays never come from the image. They appear to come from the object.

the phenomenon is known as Lateral inversion. The effect produced by a plane mirror in reversing the images from left to right and right to left while forming images is lateral inversion.

Periscope is one such device which works on the reflection of reflected light.

The light from the object falls on one mirror that is placed at 45^o to the object and the light gets reflected. This reflected light in turn falls on another mirror and is again reflected until it reaches the eyepiece.

a) The 2 plane mirrors are parallel to one another.

b) The plane mirrors make an angle of 45 degrees with respect to the sides of the tube.

Infinite number of images are formed when an object is placed between two parallel plane mirrors facing each other. The object undergoes multiple reflection to produce image of the distant object. The distance between the mirror and the object is inconsiderable in formation of infinite images. No matter how much the distance be, number of images formed will be infinite.

Kaleidoscope is the instrument that works by producing multiple reflections. It consists of 3 plain mirrors inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other.

A Kaleidoscope is a toy mostly used by children for enjoyment. It is made of a tube with plain mirrors and colored glass pieces inside it at one of the end.

Kaleidoscope having object chambers are used for hiding valuables.

A Glass Prism. It is a device used to split white light into seven colors. It has 2 triangular ends and 3 rectangular sides which help in splitting the white ray of light into a band of 7 colors, also known as spectrum.

When a beam of sunlight is passed through a glass prism, it splits up into a band of 7 colors, also known as a spectrum.

The human eye has convex type of lens. Light that hits the lens of the eye is focused by the lens so that it hits the retina. Retina has photoreceptor cells (rods and cones) which get stimulated and sends signals to the brain for viewing.

The range of vision of a normal human eye is from 25cm to infinity. The minimum distance for an object to be viewed clearly for a normal human eye is 25cm and the view may extend to infinity.

Blind spot is the point in human eye where the image is not visible. This is because there are no photoreceptors, i.e., rods and cones present in the optical disk.

The phenomenon which enables us to see movies in a cinema hall is known as Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16^th of a second, hence if 16 frames are shown on a screen in succession, they are viewed by the retina as an ongoing action.

Deficiency of Vitamin A in the diet causes night blindness.

Vitamin A helps in transforming the nerve impulses into images in the retina. Deficiency of vitamin A (also known as retinol) causes night blindness. Night blindness is the inability to see properly at night or in poor light.

Cornea is the transparent front part of an eye. It covers the front portion of the eye.

A small opening in the iris of an eye is known as pupil. Pupil controls the amount of light that enters the eye.

Iris, the part of our eye, contains pigment and gives the eye its distinctive color. Iris also helps in controlling the size of the pupil by constricting and dilating it, thus reducing and increasing the amount of light entering the eye.

The main parts of the human eye are-

IRIS- The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light reaching the retina.

RETINA- Retina receives the light focused by the lens, the photoreceptors In the retina (rods and cones) convert this light into signals and the signals are then passed onto the brain for viewing.

LENS- Lens focuses the light rays passing through it onto the retina and thus creates a clear image of the objects in sight.

PUPIL- Pupil controls the amount of light that enters the eye. It is controlled by the iris.

CORNEA- Cornea acts as the outermost layer of the eye. It protects the eye with the eyelids.

CILIARY MUSCLES- The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.

OPTIC NERVE- The optic nerve transmits signals from photoreceptors in the retina to the brain through electric impulses

In dim light the size of the pupil of our eye becomes larger or dilates to increase the amount of light going inside the eye. This dilation in size is facilitated by the muscles of iris.

In bright light the size of the pupil of our eye constricts and becomes small to reduce the amount of light going inside the eye. This reduction in size is facilitated by the muscles of iris.

(a) True. Moon is a non-luminous object. It does not have light of its own, it reflects the light of sun and it is an illuminated object.

(b) False. Diffuse reflection does not mean the failure of laws of reflection of light. Diffused reflection is caused by the irregularity of a surface.

(c) True. The pattern once seen through the eye hole can never be seen again in a kaleidoscope. The mirrors in the tube and the colored glass pieces continuously make different patterns.

a) Incidence. This is by law of reflection. The angle of incidence equals the angle of reflection.

b) 2m. Image formed by a plane mirror is far behind the mirror as the object is in front of the mirror.

c) Left ear; left hand. Images formed by a plane mirror are opposite in direction.

d) Large. The size of pupil dilates, facilitated by the iris, so that maximum amount of possible light enters the eye and a person views things clearly.

e) Fewer. Night birds have fewer cones and more rods because rods function in less amount of light. Cones function in bright light.

f) 1/16^th. The phenomenon is known as Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16^th of a second even after the object has disappeared.

g) 16. The is due to the phenomenon Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16^th of a second, hence if 16 frames are flashed in front of the eyes, they are viewed by the retina as an ongoing action

h) 24. The frame rate for motion pictures is 24 pictures per second. Frame rate is the rate at which consecutive images are displayed in a display.

The objects in a dark room cannot be seen because there is no light. We can only see something when a certain amount of light falls on it. The objects outside the room can only be seen if there is light outside the room.

Light makes things visible to us. We see an object when light passing through it is reflected.

We cannot see a book which is placed

a- Behind a wooden screen because the wooden screen is opaque so it does not allow light to pass through it.

b- In a dark room because there is no light to be reflected by the book. Thus, due to no reflection, the book is not visible.

We are able to see the moon because the light of sun falls on moon and is reflected onto the earth. Hence, we are able to see the moon.

The two types of reflection of light are Regular reflection of light and Diffused reflection of light.

Regular reflection of light makes us see an object from all directions as the reflected rays are all parallel and in a particular direction so the image of the object can be viewed from all directions. In diffused reflection of light the reflected rays are neither parallel nor in a specific direction so the image of the object isn’t visible from any direction.

Reflection of light from a wall is diffused reflection because the surface of the wall is uneven so the light is reflected in different directions and also the reflected rays are not parallel

Reflection of light from a mirror is regular reflection because the surface of mirror is an even surface which is polished. The light gets reflected from the mirror surface in one particular direction and the reflected rays are all parallel.

A book lying on a table in a room can be seen from all the parts of the room because of reflection by the light falling on the book. The light is reflected by the book and it reaches our eye at any part of the room.

i = angle of incidence and r= angle of reflection

By law of reflection, the angle of incidence is equal to the angle of reflection.

Characteristics of the image formed by a plane mirror are-

● The images formed by a plane mirror are virtual and erect

● Image formed is of the same size as the object and is laterally inverted.

● Image is far behind the mirror as the object is in front of the mirror.

A periscope is a device that reflects the reflected ray again. It consists of a tube attached to a set of mirrors. There are 2 mirrors in a periscope.

The light from the object falls on one mirror that is placed at 45^o to the object and the light gets reflected. This reflected light in turn falls on another mirror and is again reflected until it reaches the eyepiece.

● A periscope is used to see over, through or around any object.

● It can be used to see over a wall, to see around a corner.

● Periscopes are also used in warfare to keep an eye on the enemy.

The phenomenon that applies here is multiple reflection that provides the image of back of the head to be viewed. A person is made to stand in front of a mirror and another mirror is held at the back of the head. The reflection from the mirror at the back is visible in the mirror in front of the person and hence the hair at the back of the head is visible to the person in the front mirror.

The formula to calculate the no. of images of an object placed between 2 plane mirrors- (360/θ)−1; where theta is the angle of inclination.

a- 2 (360/120)-1= 3-1 =2

b- 7 (360/45)-1= 8-1 =7

c- 1 (360/180)-1= 2-1 =1

d- 5 (360/60)-1= 6-1 =5

e- 3 (360/90)-1= 4-1 =3

When 2 plane mirrors are set at right angles to each other and a coin is placed in-between these two plane mirrors, then three images will be formed, see the diagram below:

The formula for calculating the number of images, when two are kept at an angle θ, is given as: (360/θ)−1

Now,θ = 90 degree.

Thus, the number of images formed will be

= (360/θ)−1

= (360/90)-1

= 4-1

= 3

Infinite number of images of the candle will be formed because the mirrors are placed parallel to each other.

Multiple images are formed due to multiple reflection when an object is placed between two plane mirrors at any angle.

N= (360/ θ)-1

Say, the angle is 120

N= (360/120)-1

N=3-1

N=2

White light (say, sunlight) consists of seven colors can be shown by passing this light through a prism. The sunlight passes through the prism and splits into a band of 7 colors, also known as the spectrum.

The information that we get about sunlight from the formation is about dispersion of light. The separation of white light into seven different colors is known as dispersion.

Dispersion of light is the separation of white light or visible light into different colors is known as dispersion.

Formation of rainbow is a natural phenomenon which is caused by the dispersion of sunlight in the sky. The sunlight splits into 7 colors, namely, violet, indigo, blue, green, yellow, orange and red.

Kaleidoscope consists of 3 plain mirror strips which are arranged inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other.

The iris adjusts the size of pupil according to the light of varying intensity.

When the amount of light is very high, the iris contracts the pupil and reduces the amount of light entering the eye. Whereas, when the amount of light is less, the iris expands the pupil so that more light can enter the eye and things can be viewed clearly.

In bright sunshine the size of the pupil of the eye is very small so when we enter a darkened cinema hall from bright sunshine very little amount of light enters the eye. The vision improves at some time later because the pupil dilates and more amount of light enters the eye easily.

The eye lens is made up of living cells and the focal length of eye lens is adjustable. Whereas, an ordinary convex lens is made of glass and its focal length cannot be changed as it is fixed.

(a) Iris controls the amount of light entering the eye by constricting and dilating the pupil in bright and dim light respectively.

(b) Lens. The lens may diverge or converge the light rays to form an image.

(c) Retina. An upside-down image is formed on the retina.

(d) Optic nerve carries the image to brain in form of electric impulses.

(e) Ciliary muscle changes the lens shape to focus objects lying at various distances.

(a) Cones are sensitive to bright light. Cone shaped cells present in the retina are sensitive to bright light.

(b) Rods are sensitive to dim light. Rod shaped cells present in the retina are sensitive to dim light. They are most important for vision in dim light

(c) Cones are the cells of retina that produce color sensation. These photoreceptor cells are responsible for color vision.

Rods are rod shaped cells present in the retina are sensitive to dim light. They are most important for vision in dim light.

Cones are cone shaped cells present in the retina are sensitive to bright light. These photoreceptor cells are responsible for color vision.

Myopia (nearsightedness) is the defect of vision wherein a person is unable to see the distant objects clearly as the light is unable to focus on the retina and thus distant objects appear to be blurred. The problem of nearsightedness is that the light is focused in front of the retina

Myopia is corrected by wearing glasses or contact lenses. A concave lens(diverging lens) with a minus power is used in the glass, which moves the image back to retina and thus clears the image. The diverging lens will diverge the light before it reaches the retina. The light will then be converged by cornea and lens thus producing an image on the retina.

Cataract is the clouding of the lens of human eye, that lies behind the iris and pupil. It leaves to a blurred vision or decrease in vision.

The lens that has becomes cloudy is replaced by an artificial lens to restore the normal and clear vision.

- Persistence of Vision- The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16^th of a second, hence if 16 frames are shown on a screen in succession, they are viewed by the retina as an ongoing action.

One can take care of eyes by following or practicing some simple steps-

a- Watch television from a distance.

b- If a dust particle or an insect gets into our eye, we should never rub the eye, instead wash the eye with cold water.

c- Do not look at the sun directly as too much of light i.e. bright light may injure the retina.

d- Read from a normal distance of vision.

e- Never read or study in dim light or low light as it causes headaches.

If something like a dust particle or an insect gets into our eye, we should never rub the eye, instead wash the eye with cold water. If washing with cold water does not help, we should consult a doctor immediately.

5 food items rich in vitamin A are- fish, sweet potatoes, carrots, banana, pink grapefruit, apricots.

In the presence of little light the iris has to dilate pupil to facilitate entry of more and more light to view things, which leads to strain on the eyes and causes headaches.

In case of too much light, the retina may get injured and damaged thus impairing vision.

An owl can see well in the night but not during the day because owl is a night bird and night birds have fewer cones and more rods in the retina. Since rods function in less amount of light so an owl can see well in the night. To see well during the day, cones are required which are not present in adequate amount in an owl’s retina.

An eagle can see well during the day but not during the night because eagle has more cones and lesser rods in the retina. Cones function in good amount of light so an eagle can see well during the day. To see well during the night, rods are required which are not present in adequate amount in a n eagle’s retina.

Blind spot is the point in human eye where the image is not visible. This is because there are no photoreceptors, i.e., rods and cones present in the optical disk.

Night blindness is the inability to see properly at night or in poor light. In scientific terms, it is called as "nyctalopia".

The following factors cause night blindness:

1) The deficiency of Vitamin A (retinol) in the diet of a person for a considerable time can lead to the disease called night blindness.

2) The disorder in which the rod cells in the retina gradually lose their ability to respond to the light.

3) It can also happen due to cataract, or clouding of the eye’s lens.

4) Usher syndrome can also cause night blindness, which is a genetic condition that affects both hearing and vision.

5) Nearsightedness or blurred vision can also cause night blindness.

The effect produced by a plane mirror in reversing the images from left to right and right to left while forming images is lateral inversion.

Example-

The images are reversed by the plane mirror.

Objects that have their own light i.e., objects that emit light of their own are known as luminous objects. 2 luminous objects are- stars and sun.

The sun and the stars have their own light. A light bulb is also luminous.

Non- luminous objects are those that are themselves not capable of producing their own light but can reflect light from another source. 2 non luminous objects are- wood, plastics etc.

Moon also is incapable of producing its own light and it reflects sunlight and hence is non-luminous.

a- polished wooden table- regular reflection since the surface is polished (plane)

b-chalk powder-diffused reflection will occur because the surface of chalk powder is uneven

c- cardboard- diffused reflection of light since cardboard has an uneven surface

d- mirror- regular reflection since the surface of a mirror is plane

e- paper- diffused reflection since the surface of paper is uneven

f- marble floor with water spread over it- regular reflection as the floor with water will behave as a plane surface

The laws of reflection of light are-

● The incident ray, the normal ray and the reflected ray all lie in the same plane, and

● The angle of incidence is always equal to angle of reflection.

Characteristics of the image formed by a plane mirror are-

● The images formed by a plane mirror are virtual and erect

● Image formed is of the same size as the object.

● Image is laterally inverted.

● Image is far behind the mirror as the object is in front of the mirror.

Functions of parts-

IRIS- The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light reaching the retina.

EYE-LENS- Lens focuses the light rays passing through it onto the retina and thus creates a clear image of the objects in sight.

CILIARY MUSCLES- The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.

RETINA- Retina receives the light focused by the lens, the photoreceptors In the retina (rods and cones) convert this light into signals and the signals are then passed onto the brain for viewing.

OPTIC NERVE- The optic nerve transmits signals from photoreceptors in the retina to the brain through electric impulses.

According to laws of reflection, angle of reflection is always equal to the angle of incidence. The law of reflection also states that the incident ray, reflected ray and the normal to a surface, all lie in the same plane.

The images formed by a plane mirror are virtual, opposite in direction and are of the same size as the object.

The least distance of distinct vision means the minimum distance for an object to be viewed clearly. This minimum distance for a young adult with normal vision is 25cm.

The angle between the incident ray and the mirror is 30 degrees.

Angle of incidence= angle of reflection.

90 degrees is going directly into the mirror.

So, 90-30=60 degrees.

The total angle between the incident ray and the reflected ray will be 120 degree.

The image of an object formed by a plane mirror is virtual always suggesting that the light rays do not actually come from the image.

Non- luminous objects are those that are themselves not capable of producing their own light but can reflect light from another source. Moon reflects sunlight and hence is non-luminous.

Periscopes are used to see objects that are not in direct line of sight.

The relationship between no. of images and angle of mirror is

Images= (360degrees/ angle between the mirrors)-1

Images= (360 degree/90 degree)-1

Images= 4-1

Images=3

If the angle between 2 plane mirrors is decreased, the number of images of an object placed in between will increase gradually.

The relationship between no. of images and angle of mirror is

Images= (360degrees/ angle between the mirrors)-1

Vitamin A deficiencies occur from fat malabsorption, liver disorders and inadequate food intake.

Auditory nerve is not a part of the human eye, but a part of human ear. It transmits hearing information between the brain and cochlea.

See the diagram of human ear attached:

The shape of the lens is changed by ciliary muscles so that the image forms at a sharp focus at the retina. The lens becomes thicker to focus at near objects and becomes thinner to focus at distant objects.

When a person walks out of bright sunshine into a poorly lit room, the pupil dilates and becomes larger to allow maximum light to enter the eye and thus helps in viewing things easily in a poorly lit room.

Angle of incidence is always equal to angle of refection. Hence, angle of reflection will be 35°.

Others, the mirror, the screen and the polished wooden table will give regular reflection as they are even surfaces.

The image of an object is formed at the retina of an eye which is the third and the innermost coat of an eye. Light rays are focused on the retina by cornea, lens and the pupil.

The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.

The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light entering the eye.

Cataract is the clouding of the lens of human eye, that lies behind the iris and pupil. It leaves to a blurred vision or decrease in vision.

Myopia is the defect of vision wherein a person cannot see the distant objects clearly as the light is unable to focus on the retina and thus distant objects appear to be blurred.

The object distance is equal to image distance in a plane mirror.

For the distance of man from his image to be 5m-

The distance of man from mirror+ distance of image from the mirror= 5m

Since object distance= image distance

Therefore, 2*distance of man from the mirror= 5m

Distance of man from the mirror= 5/2 = 2.5m

The man is 10m from the mirror at the starting, so he will have to walk 10 – 2.5 =7.5m towards the mirror for being 5m away from his image.

The mirrors XY and YZ are at right angles, i.e., perpendicular. When a ray of light strikes the XY mirror with 65degree angle, the incident angle will be 90-65= 25 degree on the YZ mirror. Since angle of incidence= angle of reflection, the angle of reflection for YZ mirror will be 25degrees.

The phenomenon X is persistence of vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16^th of a second, hence if 16 frames are shown on a screen in succession, they are viewed by the retina as an ongoing action.

Short duration of Y is 1/16 seconds.

Z is pictures. Due to the phenomenon of persistence of vision, we are able to see moving pictures on a television or a cinema screen.

a) i-Man A suffers from hypermetropia wherein the image of a nearby object is formed behind the retina so the person has a blurred vision while looking at nearby objects.

ii- Man B suffers from myopia wherein Light from a distant object forms an image before it reaches the retina so the person can see the nearby objects clearly but not the distant objects.

b) C are Convex lens. these lenses are placed in front of a hyper-metropic eye which moves the image forward and focuses directly onto the retina which clears the vision of a nearby object.

D are concave lenses. These lenses are placed in front of a myopic eye which moves the image back to the retina and clarifies the image of a distant object.

Device P is a Kaleidoscope which consists of 3 plain mirror strips are arranged inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other.

The device Q is a Periscope. A periscope is a device that reflects the reflected ray again. It consists of a tube attached to a set of 2 mirrors. A periscope is used to see over, through or around any object. So, the student is able to see a football match clearly even when some very tall persons are sitting in front of him in the ground because the periscope helps him in seeing over, through and around the tall persons sitting in front of him in the ground.