Reflection and refraction of lightIB MYP Sciences: Revision notes
Section 1
Reflection and the law of reflection
Light is reflected when it bounces off a surface. Rays are drawn as straight lines with arrows. The normal is a line drawn at 90° to the surface at the point where the ray hits it.
Law of reflection: angle of incidence = angle of reflection, with both angles measured from the normal.
Measure angles from the normal, not from the mirror surface.
Section 2
Images in a plane mirror
A plane mirror is a flat mirror. The image it forms is:
- virtual (it cannot be put on a screen; the rays only appear to come from it)
- upright and the same size as the object
- as far behind the mirror as the object is in front
- laterally inverted (left and right swapped)
To draw a ray diagram, draw two reflected rays using the law of reflection, then extend them backwards with dotted lines to where they meet. That point is the image.
Section 3
Refraction
Refraction is the change of direction of light when it passes from one transparent material to another, caused by a change in speed.
- Air to glass or water (slows down): bends towards the normal
- Glass or water to air (speeds up): bends away from the normal
- Along the normal (angle of incidence 0°): the ray slows or speeds up but is not bent
A ray that goes through a rectangular glass block comes out parallel to the incoming ray, but shifted sideways.
Slow means towards the normal; fast means away from the normal.
Section 4
Total internal reflection and optical fibres
When light travels from a more dense material (glass) to a less dense one (air), it is refracted away from the normal. At a large enough angle of incidence, called the critical angle, no light escapes: all of it is reflected back inside. This is total internal reflection.
An optical fibre is a thin glass strand. Light is reflected repeatedly along it by total internal reflection, even round bends. Uses include endoscopes in medicine and carrying telephone and internet signals.
Section 5
Dispersion by a prism
White light is a mixture of colours. When it passes through a glass prism, each colour is refracted by a different amount, so the colours spread out into a spectrum. This is dispersion.
Red light is bent the least and violet light the most. The order is red, orange, yellow, green, blue, indigo, violet.
Must know
- Angle of incidence = angle of reflection, both from the normal
- Plane mirror image: virtual, upright, same size, same distance behind
- Refraction is caused by a change of speed: slower bends towards the normal, faster bends away
- Total internal reflection happens above the critical angle, used in optical fibres
- A prism disperses white light; red bends least, violet most
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Reflection and refraction of light
- A dentist uses a small plane (flat) mirror on a handle to look at the back of a patient's teeth. Light from a lamp reflects from the tooth, then from the mirror, and into her eye.State the law of reflection and the line from which the angles are measured.2 marks
- A student in a school laboratory shines a narrow ray of light from a ray box onto a rectangular glass block. The ray bends where it enters the glass and bends again where it leaves the glass on the other side.A second ray hits the glass block along the normal, at 0° to it. Describe what happens to the ray and explain why.2 marks
- Students in Kuala Lumpur investigate how light bends as it enters a rectangular glass block. They shine a thin ray at the block at three different angles of incidence, and measure the angle of refraction with a protractor each time. For angles of incidence of 20°, 40° and 60° the angles of refraction are 13°, 25° and 35°. Each angle was measured once.State a testable hypothesis for this investigation, and identify the independent variable and the dependent variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).