Light: rays, reflection, dispersion and the electromagnetic spectrum
Light as an electromagnetic wave, c = f·λ, the law of reflection, why prisms and raindrops make spectra, colour, and scattering. With a law-of-reflection experiment.
Read
Light is an electromagnetic wave. In a vacuum it travels at c = 3.00 × 10⁸ m/s (only slightly slower in air), and like any wave it obeys c = f·λ. Visible light has wavelengths from about 400 nm to 750 nm: red has the longest wavelength and lowest frequency, violet the shortest wavelength and highest frequency.[6]
In geometric optics light is modelled as rays travelling in straight lines, which change direction at a mirror or at the boundary between two materials. At a mirror, the angle of reflection equals the angle of incidence, both measured from the normal. A surface that is smooth compared with the wavelength gives a sharp image; a rough surface scatters light in all directions (diffuse reflection).[3][4]
White light is a fairly uniform mixture of all visible wavelengths. Because a material's refractive index depends on wavelength, each colour refracts by a different amount: this is dispersion. In glass and water the index is higher for shorter wavelengths, so violet bends more than red. A rainbow forms when sunlight refracts entering a raindrop, reflects off the back and refracts again on leaving.[5]
Light meeting matter can be transmitted, reflected, absorbed, refracted or scattered. The colour of an object is the wavelengths it reflects, while the others are absorbed.[2]
The sky is blue because molecules in the air scatter short wavelengths far more strongly than long ones. At sunset the path through the atmosphere is much longer, so most blue is scattered out of the direct beam and the Sun looks red or orange.[1]
Light from the Sun covers about 1.5 × 10¹¹ m in about 500 s: eight minutes. You see the Sun as it was eight minutes ago.[7][6]
Try it: Test the law of reflection
You need
- A plane mirror with a stand (or a block to hold it upright)
- A ray box with a single slit, or a torch with black card taped over it leaving a narrow slit
- A sheet of A4 paper, a ruler, a sharp pencil and a protractor
- A dim room
Safety
Ray boxes get hot: don't touch the bulb. If a laser pointer is used, only a low-power school laser under the teacher's supervision, and never point it at eyes or shiny surfaces facing people.
What to do
- Draw a straight line across the paper and stand the mirror on it, reflecting side along the line.
- Draw the normal: a line at 90° to the mirror line, in the middle of the page.
- Shine a narrow ray along the paper so it hits the mirror exactly where the normal meets it.
- Mark the incoming and reflected rays with two pencil dots each, then remove the mirror and draw the rays with a ruler.
- Measure the angle of incidence i and the angle of reflection r from the normal.
- Repeat for at least five angles (for example 15°, 30°, 45°, 60°, 75°) and plot r against i.
What happens, and why?
The points should lie on the straight line r = i. Small differences come from the width of the ray, the thickness of the mirror glass (the silvered back reflects, not the front) and reading the protractor. This is the law of reflection.[4]
Play: Visible or not?
Use c = f·λ (c = 3.00 × 10⁸ m/s). Is this light visible (400–750 nm), or beyond red (infrared) or beyond violet (ultraviolet)?
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Quiz
Other ages: Ages 6–7 · Ages 10–11