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Home / Physics / Determination of Focal Length of Concave Mirror and Convex Lens | Testbook

Determination of Focal Length of Concave Mirror and Convex Lens | Testbook

Last Updated on Jan 24, 2025
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Before we dive into our experiment, let's familiarize ourselves with some key terms related to a concave mirror and convex lens:

  • Centre of curvature: For a curved mirror, the centre of curvature, denoted as C, is the centre of the hypothetical glass sphere from which the mirror is a part.
  • Radius of curvature: For a curved mirror, the radius of curvature, denoted as R, is the radius of the hypothetical glass sphere that the mirror is a part of.
  • Principal axis: The principal axis is an imaginary line that passes through the pole and centre of curvature of the spherical mirror.
  • Principal focus: The principal focus is the point where the reflected rays converge or appear to converge. For a concave mirror, the principal focus is in front of the mirror, and for a convex mirror, it is behind the mirror.

Now, let's proceed with an experiment to determine the focal length of a concave mirror and a convex lens.

For more information, check out: Convex Lens

Objective

The aim of this experiment is to determine the focal length of:

  • A concave mirror
  • A convex lens by capturing the image of a distant object.

Theoretical Background

How to Determine the Focal Length of a Concave Mirror?

Here are the steps to find the focal length of a concave mirror:

  • A concave mirror is a spherical mirror whose reflecting surface is curved inward and it abides by the laws of light reflection.
  • Light rays from a distant object are considered parallel to each other.
  • If the image formed is real, inverted, and minuscule, then the parallel rays of light converge at a point in front of the mirror.
  • The image formed by the convex lens is real and can be projected onto a screen.
  • We denote the distance between the principal axis P and the focus F of the concave mirror as f.

How to Determine the Focal Length of a Convex Lens?

Here are the steps to find the focal length of a convex lens:

  • A convex lens is thicker in the middle and thinner at the edges, and it is also referred to as a converging lens.
  • The refracted rays from a parallel beam of light converge on the other side of the convex lens.
  • If the image is focused at the lens' focus, the image will be real, inverted, and very small.
  • The focal length, denoted by f, is the distance between the lens's optical centre and the principal focus.
  • The image formed by the lens is real and can be projected onto a screen.

Determining the Focal Length of a Given Concave Mirror

Materials Needed

  1. A concave mirror
  2. A measuring scale
  3. A screen holder
  4. A mirror holder
  5. A mirror stand

Procedure

  1. The distance between the selected distinct object should be more than 50 ft.
  2. The concave mirror placed on the mirror stand and the distant object should be facing each other.
  3. The screen should be placed in front of the mirror's reflecting surface. Adjust the screen to obtain a clear, sharp image.
  4. Using a metre scale, determine the distance between the concave mirror and the screen. This distance is equal to the focal length of the concave mirror.
  5. Repeat the above procedure three times to calculate the average focal length.

Experimental Setup

Observation Table

Sl.no Position of concave mirror (M) Position of screen (S) Focal length = (M-S)
1 60 cm 50 cm 10 cm
2 60 cm 50 cm 10 cm
3 60 cm 50 cm 10 cm

Calculation

Below is the mean value of the focal length of the concave mirror:

Result

The focal length of the concave mirror is 10 cm.

Precautions

  1. Ensure the distant object is well illuminated to get a distinct image.
  2. Always place the concave mirror near an open window.
  3. The polished surface of the concave mirror and the distant object should face each other.
  4. There should be no obstacles between the light rays from the object and the concave mirror.
  5. The screen and the concave mirror stand should be parallel to the measuring scale.
  6. The mirror holder along with the mirror should be perpendicular to the measuring scale.
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Determining the Focal Length of a Convex Lens

Materials Needed

  1. A wooden bench
  2. A convex lens
  3. A lens holder
  4. A screen fixed to a stand
  5. A measuring scale

Experimental Setup

Procedure

  1. Arrange the lens and screen on the wooden bench without disturbing them.
  2. Place the lens on the holder facing a distant object.
  3. Place the holder with the screen on the bench.
  4. The screen's position should be adjusted such that a sharp image of the distant object is obtained on it.
  5. The difference between the positions of the lens and the screen is equal to the focal length of the convex lens.
  6. Shift the focus to various other distant objects and calculate the focal length of the convex lens.

Observation Table

Sl.no Position of convex lens (L) Position of screen (S) Focal length = (L-S) cm
1 60 cm 50 cm f1 = 10cm
2 60 cm 50 cm f2 = 10cm
3 60 cm 50 cm f3 = 10cm

Calculation

Below is the mean value of the focal length of the convex lens:

Result

The focal length of the convex lens is 10 cm.

Precautions

  1. Ensure the convex lens is placed vertically.
  2. There should be no obstacles between the light rays from the object and the convex lens.
  3. Ensure the distant object is well illuminated to get a distinct image.
  4. The convex lens stand and the screen should be parallel to the measuring scale.
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Frequently Asked Questions

A convex lens is thicker in the middle and thinner at the edges and converges light rays, while a concave lens is thinner in the middle and thicker at the edges and diverges light rays.

The two types of spherical mirrors are concave mirror and convex mirror.

The relationship between the focal length and the radius of curvature of a concave mirror is given by f=R/2.

The mirror formula is given by 1/f = 1/v + 1/u.

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