Focal Length of a Convex Lens

To determine the focal length of a given convex lens.  

Focal Length of Convex Lens 

1. Convex lens is bulged in the centre, i.e., it is thicker in the middle and thinner at the edges. It is also called a converging lens because it converges a beam of light incident on it. 

2. When a parallel beam of light falls on a convex lens, the rays, after refraction converge at a point on its other side.

3. If the parallel beam of light comes from a distant object, a real, inverted image of a very small size is formed at the focus of the lens.

4. The distance between the optical centre of the lens ‘O’ and the principal focus ‘F’ of the lens is called the focal length of a lens, and ‘f’ is the representation of focal length.

5. Since the image formed by the lens is real, it can be obtained on a screen.

6. Types of Images: (This should be tabulated and written on the ruled side)

• Real Image 

1. It can be obtained on a screen. 

2. The light rays after reflection or refraction, actually meet at a point. E.g. Concave mirror or Convex lens forms such an image. (It is an inverted image) 

• Virtual Image 

1. It cannot be obtained on the screen. 

2. The light rays after reflection appear to diverge from a point. E.g. Plane and convex mirrors form such a virtual image. 

1. Arrange a wooden bench capable of holding the lens and screen horizontally on a table, so that the lens and screen are not disturbed. 

2. Keep the lens in a holder facing a distant object say tree branches. 

3. Fix the screen on another holder and keep it on the bench. 

4. Adjust the position of the screen in such a way that a sharp image of the given distant object falls on it. 

5. Note down the position of the lens in the table and the screen, find the differences and record the same. The difference will give the focal length of the given convex lens. 

6. Focus the lens towards various distant objects and repeat the experiment to find the position of the sharp image and thereby the focal length. 

7. Add all the focal lengths found and find the mean value of the focal length of the convex lens. 

The mean value of the focal length of the convex lens = (f₁ + f₂ + f₃ + f₄) cm / 4 = 17cm.

1. The convex lens should be placed vertically. 

2. There should be no obstacle or hurdle in the path of rays of light from the distant object incident on the convex lens. 

3. In order to get a well-illuminated and distinct image, it must be ensured that the distant object is well-illuminated. 

4. The base of the stand of the convex lens and screen should be parallel to the measuring scale.