What is light?

Light is a form of energy. This form of energy travels as a wave mostly. The light we can most relate to is what we call visible light. The form of radiant energy that stimulates the organs of sight, having for normal human vision wavelengths ranging from about 390 nm to 770 nm

But there are other wavelength of this radiant energy, we cannot see them but we can detect these forms of every and they follow the same rules.

and traveling at a speed of about 299,800 kilometres per second.

One ångstrom = 10-8 cm (0.00000001 cm).. 1nm = 10-9 m

Almost all the energy that this planet of ours has came from the Sun.


there are a number of ways that light energy can find its way into our eyes and this defines what we see.

emission: the object itself is a source of light with a color determined by its spectra

reflection: certain frequencies are reflected from the object while others are not

transmission: certain frequencies are transmitted through the object while others are not, transpanent materials ....

interference: certain frequencies are amplified by constructive interference while others are attenuated by destructive interference

dispersion: the angular separation of a polychromatic light wave by frequency during refraction, rainbow, and oil in puddles

scattering: the preferential reradiation of certain frequencies of light striking small, dispersed particles, blue sky

Speed of Light

Light moves! It moves very quickly, in fact,

Speed of light in a vacuum = 2.997925 ± 0.000002 x 108 m/s.

The first measurement of the speed of light was made by the Danish astronomer, Ole Rømer (1644–1710) about the year 1676. He observed the planet Jupiter and its satellites. Each of these satellites is eclipsed when it moves behind the planet. The time between successive eclipses of a particular satellite should be the same. Rømer found that when the earth was approaching Jupiter the eclipses became progressively earlier and that when the earth was receding from Jupiter the eclipses became progressively later.

He correctly attributed this variation to the variation in the time it took light to come from Jupiter to Earth as the distance between the two planets changed, Fig. 1.1. (The variation in the distance between Earth and Jupiter is due to the fact that Jupiter, being farther from the sun, takes much longer – approximately twelve times longer – to complete an orbit of the sun than the earth does.) Based on the information available at that time Rømer calculated a value of 2.28 × 108 m s–1 for the speed of light.

In 1849 the French physicist, Armand Fizeau (1819–1896) carried out the first terrestrial measurement of the speed of light using mirrors to make light travel a round trip of some 17 km. he used a rotating toothed wheel W, Fig. 1.2, to control the emerging and reflected light.

Fig 1.2 Fizeau’s experiment

Light from the source S was focused by a converging lens through a half-silvered plate P onto the edge of the toothed wheel. If the light was not blocked by a tooth then it passed through and travelled a distance of 8.6 kilometres to the mirror M and back to the wheel’s edge. He adjusted the speed of rotation of the wheel until he got no reflected light. He now knew that the light had travelled the round trip in the time it took the gap in the wheel to be replaced by a neighbouring tooth, blocking the light.

Fizeau calculated that light travelled at 3.15 × 108 m s–1 which was very accurate, almost within 5% of the actual value

more here

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 x Wavelength

Velocity = 2450 MHz x 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.

So how long does it take light to reach the earth from the sun? Is it instaneaneous? No because light has a speed so it does take some time. The Earth is 149,600,000 km from earth ...

how many meters is that ????

149,600,000,000 meters, but how do I find the time

speddisttime --> timedistspd

so 149600000000 / 2.998 x 108 = 499s .... in minutes ?


1. Diffraction and interference

2. Light as a transverse wave motion

3. Dispersion

4. Colours

5. Electromagnetic spectrum

6. The spectrometer

This section of the course is broken into 3 sections


Below you will find the subtopics that are on these pages

Before you draw out the ray diagrams you should probably print out the following page



1. Laws of reflection

2. Mirrors


1. Laws of refraction

2. Total internal reflection

3. Lenses

Continue on to Optic Instruments

A virtual optics bench .... check it out

Back 2 Waves

Some really nice photos here, including some that need 2-colour 3D glasses

1. Measurement of the focal length of a concave mirror.

2. Verification of Snell’s law of refraction.

3. Measurement of the refractive index of a liquid or a solid.

4. Measurement of the focal length of a converging lens.

5. Measurement of the wavelength of monochromatic light.