Light as energy

Light Travel

Spectral Dispersion



OP33 understand that light is a form of energy and that it can be converted to other forms

Light as energy

What am I thinking of?

This thing can change shape.

It can appear and disappear instantly

It can be anywhere,at anytime

Its shape can change before your eyes

Sometimes they can be scary

Sometimes they can be useful.

They can move, but they are not alive

Without light, They simply do not exist

sources and transmission of light; speed of light; shadows; colour and the visible spectrum; dispersion of white light reflection of light at plane surfaces; image in a plane mirror refraction; refraction by lenses applications of reflection and refraction

OP34 show that light travels in straight lines and explain how shadows are formed

OP35 understand that luminous objects are a source of light while non-luminous objects are seen as a result of light reflected from them


A Luminous object is one that gives out its own light

A non-luminous object we see because other light reflects from it.

Make a very complete list of Luminous objects!

What is everything else ?

Transparent .... light travels straight through a medium

Translucent ... light travels through but not straight and thus we cannot 'see' through such materials

Opaque .... light does not travel through such objects

OP36 recall that white light is made up of different colours which can be separated by dispersion.

OP37 produce a spectrum of white light using appropriate apparatus, and list the colours of the spectrum

Spectral Dispersion

OP38 investigate the reflection of light by plane mirrors, and illustrate this using ray diagrams; demonstrate and explain the operation of a simple periscope


OP39 show the refraction of light as it passes from: air to glass, air to water, glass to air, water to air; show refraction of light through a lens; demonstrate the operation of a magnifying glass


A site with some experiments to try at home with optic equipment in mind

Speed of Light

Finding the Speed of Light with

Marshmallows-A Take-Home Lab

Robert H. Stauffer, Jr., Cimarron-Memorial High School, Las Vegas, Nevada, USA

I have heard that at 16 years old, Albert Einstein constantly wondered what it would be like to ride on a beam of light. Students in physics always seem to be fascinated by the properties of light. However, speed-of-light demonstrations often require extensive preparation or expensive equipment. I have prepared a simple classroom demonstration that the students can also use as a take-home lab.

The activity requires a microwave oven, a microwave-safe casserole dish, a bag of marshmallows, and a ruler. (The oven must be of the type that has no mechanical motion-no turntable or rotating mirror. If there is a turn-table, remove it first.) First, open the marshmallows and place them in the casserole dish, completely covering it with a layer one marshmallow thick. Next, put the dish of marshmallows in the microwave and cook on low heat. Microwaves do not cook evenly and the marshmallows will begin to melt at the hottest spots in the microwave. (I leaned this from our Food Science teacher Anita Cornwall.) Heat the marshmallows until they begin to melt in four or five different spots. Remove the dish from the microwave and observe the melted spots. Take the ruler and measure the distance between the melted spots. You will find that one distance repeats over and over. This distance will correspond to half the wavelength of the microwave, about 6 cm. Now turn the oven around and look for a small sign that gives you the frequency of the microwave. Most commercial microwaves operate at 2450 MHz.

All you do now is multiply the frequency by the wavelength. The product is the speed of light.


Velocity = Frequency ´ Wavelength

Velocity = 2450 MHz ´ 0.122 m

Velocity = 2.99 ´ 108 m/s

This works in my physics class, often with less than 5% error. Then the students can eat the marshmallows.