A chef places a tray of food into an industrial oven that uses infrared radiation to cook the food. The tray is made from a shiny metal surface. Explain why the shiny metal tray reflects most of the infrared radiation incident on it, and describe what would happen to the frequency and wavelength of the infrared waves as they pass from the air into the denser glass viewing window of the oven door.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Infrared radiation is used in industrial ovens to heat food. The oven door contains a thick glass viewing window. The metal tray inside the oven has a smooth, shiny surface.
Model answer (5 marks)
The shiny, smooth metal surface causes specular reflection, so the angle of incidence equals the angle of reflection (law of reflection). When the infrared waves enter the denser glass viewing window, their frequency remains constant. However, the speed of the waves decreases in the glass. Because the frequency is unchanged and the speed is reduced, the wavelength must decrease.
Examiner tips
- Mention specular reflection and the law of reflection for the metal tray.
- State that frequency is unchanged when moving between media.
- Explain that speed decreases in glass, leading to a shorter wavelength.
Common mistakes
- Saying the metal absorbs the infrared instead of reflecting it.
- Claiming the frequency changes when entering glass.
- Ignoring the relationship between speed, frequency and wavelength in the new medium.
Mark scheme (5 marks)
- The shiny/smooth metal surface causes specular reflection
- The angle of incidence equals the angle of reflection (law of reflection)
- The frequency of the infrared waves remains constant / does not change as the waves enter the glass
- The speed of the infrared waves decreases as they enter the denser glass medium
- Because frequency is constant and speed decreases, the wavelength also decreases
Key terms in this question
infrared radiation · frequency · wavelength
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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