Common mistakes in IGCSE Physics waves and light

The distance from a crest to a trough is 8.0 cm. What is the amplitude?

✗ Wrong: Amplitude = 8.0 cm

✓ Right: Amplitude = 4.0 cm. It is measured from the middle line up to a crest.

Describe a longitudinal wave.

✗ Wrong: The vibrations are at right angles to the direction the wave travels, like light.

✓ Right: The vibrations are parallel to the direction the wave travels, like sound.

Light passes from glass into air. Which way does it bend?

✗ Wrong: Towards the normal.

✓ Right: Away from the normal, because it speeds up. Slowing down bends light towards the normal.

State the conditions for total internal reflection.

✗ Wrong: The angle must be bigger than the critical angle.

✓ Right: Light must travel from the denser (slower) material towards the less dense one, for example glass to air, AND the angle of incidence must be greater than the critical angle.

An echo returns 2.0 s after a shout. Sound travels at 340 m/s. How far away is the wall?

✗ Wrong: distance = 340 × 2.0 = 680 m

✓ Right: Total path = 340 × 2.0 = 680 m. The wall is half of that: 340 m.

Which electromagnetic wave has the shortest wavelength?

✗ Wrong: Radio waves, because they have the lowest frequency.

✓ Right: Gamma rays. All electromagnetic waves travel at the same speed in a vacuum, so from v = fλ the highest frequency has the shortest wavelength.