Polarisation & EM Spectrum

Overview

This lesson links two parts of the wave model: electromagnetic waves are transverse, and transverse waves can be polarised. You should also know the main regions of the electromagnetic spectrum and use Malus’s law for plane-polarised light.

What You Need to Know

• All electromagnetic waves are transverse and travel at speed c in free space.
• The main spectrum order by decreasing wavelength is radio, microwave, infrared, visible, ultraviolet, X-rays, and gamma rays.
• Visible light has wavelengths of about 400-700 nm in free space.
• Polarisation means the oscillations of a transverse wave are restricted to one plane.
• Longitudinal waves cannot be polarised because their oscillations are along the direction of travel.
• For plane-polarised electromagnetic waves passing through a polarising filter, use I = I0 cos^2(theta).

How to Work Through It

1. Recall the electromagnetic spectrum order and the visible wavelength range.
2. Use transverse-wave diagrams to show the plane of oscillation.
3. Compare one polarising filter with two filters at different angles.
4. Practise Malus’s law calculations, including angles of 0 degrees, 45 degrees, and 90 degrees.

Check Your Understanding

• Why does polarisation show that light is transverse?
• What happens to transmitted intensity when the filter angle is 90 degrees to the polarisation direction?
• Which end of the electromagnetic spectrum has the shortest wavelength?
• What wavelength range is visible to the human eye?

Common Mistakes

• Saying that polarisation changes the speed of the wave.
• Applying Malus’s law to unpolarised light when the question only covers plane-polarised light.
• Mixing up wavelength order and frequency order in the electromagnetic spectrum.
• Forgetting that the angle in Malus’s law is between the plane of polarisation and the filter axis.

Next Steps

• Memorise the electromagnetic spectrum order and the visible wavelength range.
• Practise Malus’s law calculations with clear angle labels and units.