Question

1-What is the expression for the thin lens equation? Define all the terms in the expression.

2-Describe the three properties that determine the focal length of a lens.

3-Define the following:
i. Converging lens______

ii. Diverging lens_______

iii. Focal length___________

iv. Focalpoint____________

v. Object distance_______

vi. Imagedistance___

vii. Real image___

viii. Virtual image

Answer 1

1. The expression for Thin Lens Formula is

where u= object distance

v=  image distance

f =  focal length.

2. The three properties that determine the focal length of a lens are -

a) Refractive index of the lens material

b) The radius of curvature of the surfaces of the lens

c)  The medium in which the lens resides.

3. (i) A converging lens (or convex lens) is curved outwards on both sides. These lenses are thicker in the middle. Rays of light that pass through the lens are brought closer together (they converge). When parallel rays of light pass through a converging lens, the refracted rays converge at the focal point.

(ii)  A diverging lens (or concave lens) is curved inwards on both sides. These lenses are thinner in the middle.  Rays of light that pass through the lens are spread out (they diverge). When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus.

(iii)  Focal length is the distance between the Pole of the lens and the point where the parallel incident rays meet ( or intersect).

(iv)  The Focal Point also called  Principal Focus is a point on the principal axis of a spherical mirror or of a lens. Any light source which is kept at the Focal point allows the rays to pass parallel through the principal axis after being reflected or refracted from the spherical mirror or lens respectively. The distance between the Pole and Focal point is called the focal length.

(v) Object distance is the distance between the Pole of the mirror/lens and the object placed.

(vi) Image distance is the distance between the Pole of the mirror/lens and the image formed after reflection or refraction.

(vii) A real image is formed when the rays of light after reflection or refraction actually meet at some point. It is formed by the actual intersection of light rays. It can be caught on a screen. A real image is always inverted.

(viii) A virtual image is formed when the rays of light after reflection or refraction appear to meet at a point. It  is formed by the imaginary intersection of light rays. It can not be caught on a screen rather it can be taken on a mirror. A virtual image is always erect.