Light
Reflection at plane mirrors, refraction and refractive index, and the images formed by a thin converging lens.
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Before you begin
Light travels in straight lines until it meets a mirror or the boundary of a new material. The lessons start with the law of reflection at plane mirrors, then explain why light changes direction as it enters glass or water and calculate the new angle using refractive index. They finish with converging lenses: how a lens brings a beam to a focus, how to measure its focal length, and how to describe the images it forms.
Be comfortable with: wave speed, because refraction happens when the speed of light changes.
Learning goals
- Use the normal, angle of incidence and angle of reflection
- Apply the law of reflection in constructions, measurements and calculations
- Use the normal, angle of incidence and angle of refraction
- Apply sin i divided by sin r as a constant for a fixed pair of media
- Define refractive index as vacuum light speed divided by medium light speed
- Describe how a thin converging lens acts on a light beam
- Define the focal length of a converging lens
- Describe lens images as real or virtual, magnified or diminished, and upright or inverted
Syllabus statements covered
- recall and use the terms normal, angle of incidence and angle of reflection to describe the reflection of light
- state that, for reflection, the angle of incidence is equal to the angle of reflection and use this principle in constructions, measurements and calculations
- recall and use the terms normal, angle of incidence and angle of refraction to describe the refraction of light
- recall and apply the relationship sin i / sin r = constant to new situations or to solve related problems
- define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium
- describe the action of a thin converging lens on a beam of light
- define the focal length for a converging lens
- describe the characteristics of images (e.g. real / virtual, magnified / diminished, and upright / inverted) formed by a thin converging lens (recall of the characteristics of images formed for specific object distances is not required).
Lessons
Work through them in order.
- Reflection of lightMeasure angles from the normal and use the law of reflection to construct, measure and calculate ray paths at plane mirrors, including a periscope.
- Refraction at a boundaryExplain why light bends at a boundary and predict whether a ray bends towards or away from the normal.
- Refraction calculationsUse a sine graph to distinguish angle ratios from sine ratios, find an unknown angle, and connect refractive index to light speed.
- Converging lenses and focal lengthDescribe a converging lens acting on a parallel beam and estimate focal length safely with a distant object.
- Describing lens imagesUse ray paths, screen evidence, heights and orientation to describe lens images.
Practise and check
Topic reference
Quick reference
| Idea | Relationship or rule | Note |
|---|---|---|
| law of reflection | angle of incidence = angle of reflection, i = r | both angles measured from the normal |
| angle to the surface | i = 90°- angle between the ray and the surface | for a mirror or a boundary |
| refraction | (sin i)/(sin r) = constant | the sines, not the angles, are in proportion |
| refractive index | n = c/v | c = 3.00 × 10⁸ m/s; no unit; greater than 1 |
| into a slower material | ray bends towards the normal | for example, air into glass or water |
| into a faster material | ray bends away from the normal | for example, glass or water into air |
| focal length | distance from the optical centre to the principal focus | measure with a distant object |
Core knowledge to remember
- The normal is a line at 90° to the mirror or boundary at the point of incidence. The angle of incidence is the angle between the incident ray and the normal; the angle of reflection and the angle of refraction are measured from the normal in the same way.
- For reflection, the angle of incidence is equal to the angle of reflection.
- For refraction between two materials, sin i/sin r is a constant.
- The refractive index of a medium is the ratio of the speed of light in a vacuum to the speed of light in the medium, n = c/v.
- A thin converging lens brings a beam of light parallel to its principal axis to a point, the principal focus.
- The focal length of a converging lens is the distance between its optical centre and its principal focus.
- An image is described as real or virtual, magnified or diminished, and upright or inverted. A real image can be caught on a screen; a virtual image cannot.
Common mistakes
- Measuring angles from the mirror or boundary surface instead of from the normal.
- Dividing the angles, i/r, instead of their sines.
- Saying a larger refractive index means light travels faster.
- Saying light always bends towards the normal. It bends away when it speeds up.
- Giving a scale reading as the focal length instead of a distance from the lens.
- Saying a real image must be diminished, or that a converging lens always gives an inverted image.