Properties of electromagnetic waves — Required practical 10

EM waves can be:

• Reflected at boundaries (mirrors for visible, satellite dishes for microwaves).
• Refracted — bent when entering a different medium.
• Absorbed — some materials absorb specific wavelengths.
• Transmitted — passes through.

Refraction

When a wave passes from one medium to another with different propagation speed, its direction changes (unless it hits the boundary normally).

• Going from less dense to denser medium (e.g. air → glass): light slows down and bends toward the normal.
• Going from denser to less dense: light speeds up and bends away from the normal.
• Wavelength changes; frequency stays constant.

Snell's law (HT)

$n_1 \sin\theta_1 = n_2 \sin\theta_2$

(not always required at GCSE — graphs of refraction are common.)

Refraction in a glass block

• Incident ray bends toward normal entering glass.
• Travels through glass at slower speed.
• Bends away from normal on exit.
• Often emerges parallel to incident, but laterally displaced.

Required practical 10 — IR absorption and emission

Investigates how surface colour affects:

• Emission of infrared from hot surfaces.
• Absorption of infrared by cool surfaces.

Apparatus: Leslie cube (metal cube with different colored faces — black, white, shiny, dull) filled with hot water; infrared detector or thermopile.

Method:

• Fill cube with hot water.
• Hold detector at the same distance from each face.
• Record reading for each face.

Result: black, dull surfaces emit and absorb IR best. Shiny, light surfaces reflect best.

Implications

• Survival blankets are silvery to reflect body heat.
• Cooling fins on radiators are black to maximise emission.
• Houses with light roofs stay cooler in hot climates.