How do waves reflect and refract, and how do lenses form images?
The reflection of waves and the law of reflection, the refraction of waves at a boundary and why it happens, total internal reflection, and how converging and diverging lenses form images.
A focused answer to OCR Gateway GCSE Physics A topic P5 on reflection, refraction and lenses, covering the law of reflection, the refraction of waves at a boundary and why it happens, total internal reflection, and how converging and diverging lenses form images.
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What this topic is asking
OCR wants you to apply the law of reflection, explain refraction at a boundary and why it occurs, describe total internal reflection, and explain how converging and diverging lenses form images. This is topic P5.3 of the OCR Gateway Physics A (J249) specification, linked to the reflection and refraction practical.
Reflection
This law applies to all waves, including light, water waves and sound (an echo). A plane mirror forms an image that is the same size as the object, upright, and the same distance behind the mirror as the object is in front (a virtual image).
Refraction
The reason is that the part of the wavefront that crosses the boundary first changes speed first, so the wavefront pivots, like a car driving from road onto mud at an angle. This is why a straw in a glass of water looks bent at the surface.
Total internal reflection
Below the critical angle the light mostly refracts out; at the critical angle it travels along the boundary; above it, all the light reflects.
Lenses
A converging (convex) lens is thicker in the middle and brings parallel rays of light together to a focus. It can form a real, inverted image (which can be projected onto a screen) of a distant object, as in a camera or projector, or a magnified virtual image when used as a magnifying glass with the object close up.
A diverging (concave) lens is thinner in the middle and spreads light out as if it came from a focus behind the lens. It always forms a virtual, upright, smaller image and is used to correct short-sightedness.
Try this
Q1. State the law of reflection. [1 mark]
- Cue. The angle of incidence equals the angle of reflection (measured from the normal).
Q2. State what happens to the speed of light when it passes from glass into air. [1 mark]
- Cue. It speeds up (and bends away from the normal).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksA ray of light passes from air into a glass block. Describe what happens to the direction and speed of the light as it enters the glass, and explain why this change of direction occurs.Show worked answer →
A P5 question worth four marks on refraction. As the light enters the glass it slows down because glass is optically denser than air (1 mark). It changes direction, bending towards the normal (the line at right angles to the surface) as it enters the denser medium (1 mark). The change of direction (refraction) happens because the wave changes speed at the boundary (1 mark): the part of the wavefront that enters the glass first slows first, so the wavefront pivots and the ray bends (1 mark). Markers reward slowing down, bending towards the normal, and the link between the change of speed and the change of direction. A common error is to say light speeds up entering glass.
OCR 20213 marksDescribe how a converging (convex) lens forms a real image of a distant object, and state one use of a converging lens.Show worked answer →
A P5 question worth three marks. A converging lens brings parallel rays of light from a distant object together to a focus, forming a real, inverted image that can be projected onto a screen (2 marks for converging the rays to a real, inverted image). A use of a converging lens is in a camera, a projector, a magnifying glass, or to correct long-sightedness (1 mark). Markers reward the converging of light to a real inverted image and a valid use. A common error is to confuse a converging (convex) lens with a diverging (concave) lens, which spreads light out and forms a virtual image.
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