NSAA NSAA Physics - Waves and Optics 3

Question 1

A convex lens of focal length 20 cm forms a real image of an object 60 cm away. How far from the lens is the image?

Accepted Answer

30 cm

Answer

1/f = 1/v - 1/u (real is positive): 1/20 = 1/v + 1/60 → 1/v = 3/60 - 1/60 = 2/60, v = 30 cm.

Question 2

What is the intensity relationship between two points at distances r₁ and r₂ from a point source?

Accepted Answer

I₁/I₂ = (r₂/r₁)²

Answer

Intensity from a point source follows the inverse square law: I ∝ 1/r², so I₁/I₂ = (r₂/r₁)².

Question 3

When light travels from a medium with refractive index 1.5 into air (n=1), what is the critical angle?

Accepted Answer

41.8°

Answer

sin(θc) = n₂/n₁ = 1/1.5 = 0.667, so θc = arcsin(0.667) ≈ 41.8°.

Question 4

Which wave property allows sound to be heard around corners?

Accepted Answer

Diffraction

Answer

Diffraction allows waves to bend around obstacles and spread out through gaps.

Question 5

If the amplitude of a wave is doubled, the intensity of the wave:

Accepted Answer

Quadruples

Answer

Intensity is proportional to amplitude squared (I ∝ A²), so doubling amplitude quadruples intensity.

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