Question

A convex lens of diameter 8.0 cm is used to focus a parallel beam of light of wavelength 620 nm. If the light is focused at a distance of 20 cm from the lens, what would be the radius of the central bright spot formed?

Answer 1

Given:

Wavelength of the light used,

λ=620 nm=620×10⁻⁹m

Diameter of the convex lens,

d=8 cm=8×10⁻² m

Distance from the lens where light is to be focused,

D=20 cm=20×10⁻²m

The radius of the central bright spot is given by

R=1.22λD/d

=1.22×620×10⁻⁹×20×10⁻²/8×10⁻²  

= 1891×10⁻⁹ m

≈1.9×10⁻⁶m

∴ Diameter of the central bright spot, 2R = 3.8 × 10⁻⁶ m.

Answer 2

Answer:

A convex lens of diameter 8:0 cm is used to focus a parallel beam of light of wavelength 620 nm. If the light be focused at a distance of 20 cm from the lens, what would be the radius of the central bright spot formed