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2 Investigating the reflection of visible light - ray box experiments and mirrors

When light waves meet the boundary between two materials, some of the wave energy might be reflected, absorbed or transmitted (and then maybe refracted).

Light waves are readily reflected off smooth flat surfaces e.g. light reflected off a mirror.

Some of the light might also be absorbed by the surface.

Set out a white sheet of paper with a line marked on it, as shown in the above diagram.

Draw a 'normal' at 90^o (perpendicular) to this line.

The normal is an imaginary line where the light ray hits the 'surface' and helps construct ray diagrams

Place the mirror adjacent to this line at 90^o to the normal.

You need a light box with a slit to give a narrow beam of light.

Place the light box on the sheet of white paper so the beam of light shines onto a mirror at the point on the mirror where the previously marked normal line is.

Mark out a series dots on the white paper coincident with the thin rays of light for the incident ray and reflected ray.

You can then join the dots up and measure the angle of the incident ray and reflected ray with respect to the normal with a protractor (NOT with respect to the plane mirror surface).

The experiment is best done in a darkened room and make sure the light beam skims over the surface of the paper.

You repeat the experiment and changing the angle of incidence (i on the diagrams), so you then also change the angle of reflection (r on the diagrams).

For a fair test use the same mirror and ray box beam to keep any variables constant.

Hopefully you see the reflected ray as thin and bright as the incidence ray - a quality plane mirror should give a clear reflection with little if any of the light absorbed.

Typical results are described, analysed and explained below.

Reflection ray diagram

Reminder - the vertical dotted line is called the 'normal', it isn't a ray, but helps in the construction and interpretation of ray diagrams.

All angles are measured with respect to the 'normal' which is at 90^o to the reflective surface.

A plane mirror means one with a perfectly flat surface.

Angle 2 = angle i is defined as the angle of incidence of incident ray.

Angle 3 = angle r is defined as the angle of reflection of the reflected ray.

You will always find hat Angle 2 = Angle 3, angle of incidence equals the angle of reflection for a plane mirror.

This is the law of reflection.

 

The law applies to all mirrors whatever the shape of the reflecting surface

The convex mirror disperses the rays and the concave mirror concentrates the rays at specific focus point (see reflecting telescope).

So, appreciate that the reflection rule (angle i = angle r) applies whatever the shape of the mirror!

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Check out your practical work you did or teacher demonstrations you observed, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

experiments - investigation of reflection using a ray box

reflecting light off a plane mirror at different angles

INDEX notes: Light reflection and uses of plane & curved mirrors

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Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

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Keywords, phrases and learning objectives for the reflection of visible light rays

Know how to use a ray box to Investigate the reflection of visible light by plane mirrors, concave curved mirrors and convex curved mirrors and be to explain their behaviour in terms of angle of incidence and angle of reflection.

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INDEX of physics notes on light reflection and mirrors

INDEX of all notes on waves, radiation, astronomy etc.

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INDEX notes: Light reflection and uses of plane & curved mirrors