# Lenses

Question 1 Define optical centre?

Question 2 Define principal focus?

Question 3 Define focal length?

Question 4 Define principal foci?

Question 5 Define first focal plane?

Question 6 Define first principal focus?

Question 7 Define second principal focus?

Question 8 What do you mean by aperture of lens?

Question 9 What are converging lenses?

Question 10 what are diverging lenses?

Question 11 Define second focal plane?

Terms used in lenses

1)Principal Axis

2)Optical centre(o):A point in or out of lens through which ray of light pass undeviated.

3)Principal Focus(F):A point on principal axis where all rays of light parallel to principal axis meet or appear to meet after passing through lens.

4)Focal length(f):The distance between the principal focus and optical centre of a lens is called focal length of lens.

5)Principal foci:A beam of light may fall on either side of a lens therefore a lens has two principal foci.

6)First principal foci(F1 ):The position of a point on the principal axis of a lens so that the ray of light starting from this point after passing through the lens travel parallel or appear to travel parallel to principal axis.)

A vertical plane passing through first principal focus is called first focal plane.

The distance between optical centre and first principal focus is called first principal focal length.

7)Second principal focus:The position of a point on principal axis of a lens where a beam of light parallel to principal axis meets or appear to meet after passing through the lens.

8)Second Focal plane:A vertical line passing through second principal focus.

The distance between optical centre and second principal focus is called second principal focal length.

9)Aperture of lens:The part of lens exposed to incident light.

A convex lens converges ray of light so it is called as converging lens.

A concave lens diverges ray of light so it is called as diverging lens.