When light crosses from air into glass, its speed decreases and its frequency stays the same. Since wavelength equals speed divided by frequency, the wavelength decreases too. Use the speed for the medium where you want the wavelength.
The 2027 SEC G3 Physics K323 syllabus covers the wave equation and refraction. This guide uses two speeds for one light source, then considers what happens when the light leaves the glass.
- Use v = fλ to connect the three wave quantities.
- Keep the frequency fixed at a stationary boundary.
- Use the wave speed in the medium named in the question.
- Compare the wavelengths using the speed ratio.
CHAPTER 1 OF 4 · Back to contents
Follow the crests across the boundary
A steady source sends out a fixed number of wave cycles per second. At a stationary boundary, the transmitted wave has that same frequency. The spacing between crests changes when the speed changes.
Use λ = v/f. If the speed becomes two thirds of its original value while frequency stays fixed, the wavelength also becomes two thirds of its original value.
Wavelength is a distance. Frequency is a count of cycles per second in hertz. Keeping their units visible helps prevent swapping them.
CHAPTER 2 OF 4 · Back to contents
Worked example | Light travelling from air into glass
Light has frequency 6.0 × 10¹⁴ Hz. Use a speed of 3.0 × 10⁸ m/s in air and 2.0 × 10⁸ m/s in the glass. Calculate its wavelength in each medium.
| Medium | Speed | Wavelength |
|---|---|---|
| Air | 3.0 × 10⁸ m/s | 5.0 × 10⁻⁷ m |
| Glass | 2.0 × 10⁸ m/s | 3.3 × 10⁻⁷ m to two significant figures |
In air, divide 3.0 × 10⁸ by 6.0 × 10¹⁴ to get 5.0 × 10⁻⁷ m. In glass the same division with the lower speed gives about 3.3 × 10⁻⁷ m.
The unrounded glass wavelength is two thirds of the air wavelength. This is a useful check on the powers of ten.
Contents · Previous chapter · Next chapter
CHAPTER 3 OF 4 · Back to contents
Practise following light back into air
If the light leaves the glass and enters air again, its speed returns to the air value in this model. The frequency stays fixed and the wavelength returns to 5.0 × 10⁻⁷ m.
Even when a ray meets the surface along the normal and continues straight, its speed and wavelength can change. The direction stays the same in that case.
If a question asks for a comparison without a numerical frequency, use the ratio of speeds. The common frequency cancels.
Contents · Previous chapter · Next chapter
CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Do I use the speed in air for the glass wavelength?
Use the speed in glass for the wavelength in glass.
What is the air wavelength in nanometres?
It is 500 nm because one nanometre is 10⁻⁹ m.
Does the frequency change again on leaving the glass?
It stays the same as the light crosses the stationary boundary.
Contents · Previous chapter · Continue with the related guide