Question

1. What is meant by the focal length of a lens or a mirror? What is the difference between a real image and a virtual image?

2. What are principal rays? Sketch them and explain each.

Answer 1

Focal Length -

The focal length of an optical system is a measure of how strongly the system converges or diverges light. If the Focal length is short ,then the rays are bended more sharply as compared to Long Focal length.

Focal Length for a thin lens is given by -

1/f = 1/di + 1/do
f = Focal Length of Lens
do = Distance of Object from lens.
di  = Distance of image from lens.

For a converging lens (for example a convex lens), the focal length is positive.
For a diverging lens (for example a concave lens), the focal length is negative.

For a spherically curved mirror in air, the magnitude of the focal length is half the radius of curvature of the mirror.

f = R/2
The focal length is positive for a concave mirror, and negative for a convex mirror.

Difference between a real image and a virtual image -

• Real Image can be actually formed on Screen.
• Light Rays After Refraction meet .
• Real Image is Inverted.

• Virtual Image cannot be formed on Screen.
• Light Rays Do not meet after Refraction .
• Virtul Image is Erect.

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