Focal Length Of Concave Mirror And Convex Lens

MATERIALS REQUIRED

A concave mirror, a metre scale, a mirror holder.

THEORY

1. A spherical mirror whose reflecting surface is curved inwards, i.e., faces towards the centre of the sphere is called a concave mirror.

2. It obeys the law of the reflection of light like a plane mirror.

3. Thus, the parallel beam of a ray of light coming from a distant object, such as the sun or a building can be considered as parallel.

4. When the parallel ray is incident on the reflecting surface of a mirror, then after reflection, the rays converge at a point, and this point is called principal focus of the concave mirror as shown in the figure.

5. since the image formed by the mirror is real so that it can be obtained on the screen.

6. The distance between the pole O and principal focus F of a concave mirror is called the focal length of the mirror. It is equal to half the radius of curvature of the mirror.

PROCEDURE

1. Choose a distant object like a tree or the sun to at as an object for our experiment.

2. Mount the concave mirror in a mirror holder.

3. Adjust the concave mirror in such a way that the rays of light coming from the tree fall directly on its reflecting surface.

4. Measure the horizontal distance between the wall and the centre of a concave mirror with the help of a meter scale.

OBSERVATION AND CALCULATION

Mean Focal length = 18 cm

RESULT

The approximate focal length of the given concave mirror is 18 cm as determined by the above method.

PRECAUTIONS

1. The mirror should be placed vertically in the lens holder.

2. There should not be any obstacle in the path of rays of light incident on the concave mirror

3. The meter scale must be correctly positioned between the wall and centre of the concave mirror.

MATERIALS REQUIRED

A convex lens, a lens holder, a white screen such as a wall or white painted board, meter scale, distant object.

THEORY

PROCEDURE

1. Choose any distant object like a tree or the sun as an object for the experiment.

2. Held the lens vertically inside a lens holder and lens must be kept in vertical position during the experiment.

3. Place a screen on the other side of the lens.

4. Obtain the image of tree/building on a white screen or wall. Move the lens to get a sharp image.

5. Using a meter scale, measure the distance from the lens to the screen.

OBSERVATION AND CALCULATION

Mean value of the focal length = 13.25 cm

RESULT

PRECAUTIONS

1. The lens should be kept vertical inside the lens holder.

2. While measuring the distance, the distance between the sharp image and the centre of the lens is to be measured.

3. The lens the screen must be at the same level.

The spherical mirror is a part of a hollow sphere with one side having silver/mercury coating, further coated with paint to protect it from damage.

It is of two types :

1. Concave mirror: Silvered at the outer surface so that reflection takes place from the inner surface(concave).

2. Convex mirror: Silver at inner surface so that reflection takes place from the outer surface(convex).

It is the width of the reflecting surface from which reflection takes place.

, The centre of curvature, is the geometrical centre of the hollow sphere “c” from which the mirror is formed.

The radius of curvature is the radius “R” of the hollow sphere whose part is a spherical mirror.

The Principal axis is the straight line joining the pole and centre of curvature.

The principal focus is the point “f” on the parallel axis where a parallel beam of light actually meets after reflection.

The brightest and the sharpest image for the concave mirror is formed at the focus.

1. Virtual, erect and enlarged image of an object is formed behind the concave mirror.

2. The real and inverted image is formed in front of the mirror.

A concave mirror is called a converging mirror because it converges the parallel beam of light ray after reflection at a point.

The uses of concave mirror are:

1. They are reflectors in the torch to reflect the light rays.

2. It is used as a shaving mirror to get an erect and enlarged image of the face.

A real image is obtained on the screen, they are formed by the interaction of a ray of light after reflection.

A virtual image is formed when the ray of light appear to meet after the reflection; Image cannot be taken on the screen.

1. The angle incidence is equal to the angle of reflection.

2. The incident ray, the reflected ray and the normal at the point of incidence,all lie on the same plane.

A homogenous transparent material or medium bounded by two surfaces at different or same radii of curvature is called a lens.

1. Double convex lens: if both the refracting surface is convex, it is thicker at the middle and thinner at the edges.

2. Double concave lens: If both the refracting surface of the lens is concave, it is thinner at middle and thicker at the edges.

Lens formula:

Where f is the focal length of the lens

u is the distance between the object, and the lens

v is the distance between the image and the lens.

The mirror formula:

Where f is the focal length of the mirror

v is the distance between the image and lens

u is the distance between the object and the mirror.

The lens will behave like a plane glass only when it is kept in a medium whose refractive index is equal to that of the lens .

The magnification power of the lens can be increased by using a number of lenses.

1. Convex lens, magnification is positive for virtual image and negative for real image.

2. Concave lens, magnification is always positive because virtual image is always formed.

The power lens is the degree of convergence or divergence of the light ray incident on any refracting surface of the lens.

the magnification power of the lens is inversely proportional to the focal length of the lens.

A full-size image is formed, but only the intensity or brightness of the image will reduce.