The Visible Spectrum and Dispersion

The two most obvious properties of light are readily describable in terms of the wave theory of light: intensity (or brightness) and colour. The intensity of light is the energy it caries per unit time, and is related to the square of the amplitude of the wave, just as for any wave (1;…). Visible light – that to which our eyes are sensitive – falls in the wavelength range of about 400 nm to 750 nm. This is known as the visible spectrum, and within it lie the different colors from violet to red. Light with wavelength shorter than 400 nm is called ultraviolet (UV), and light with wavelength greater than 750 nm is called infrared (IR) (2;...).

A prism separates white light into a rainbow of colours. This happens because the index of refraction of a material depends on the wavelength. White light is a mixture of all visible wavelengths, and when incident on a prism, the different wavelengths are bent to varying degrees. Because the index of refraction is greater for the shorter wavelengths, violet light is bent the most and red the least (3;…).

Rainbows are a spectacular example of dispersion – by drops of water (4;…). Red and violet rays are bent by spherical water droplets and are reflected off the back surface. Red is bent the least and so reaches the observer’s eyes from droplets higher in the sky. Thus, the top of the rainbow is red.

Diamonds achieve their brilliance from a combination of dispersion and total internal reflection (5;…). Incident light therefore strikes many of the internal surfaces before it strikes one at less than 25° and emerges. After many such reflections, the light has traveled far enough that the colours have become sufficiently separated to be seen individually and brilliantly by the eye after leaving the crystal.

A. This spreading of white light into the full spectrum is called dispersion.

B. You can see rainbows when you look at falling water droplets with the Sun at your back.

C. White light passes through a slit and an interference pattern is observed on a screen.

D. The colour of the light is related to the wavelength or frequency of the light.

E. Because diamonds have a very high index of refraction of about 2,3, the critical angle for total internal reflection is only 25°.

F. Although human eyes are not sensitive to UV or IR, some types of

photographic film do respond to them.

B. Choose the correct item to fill in the gaps

1. When a crest of one wave arrives at the same time as a crest of another wave, the amplitudes of two waves add to form a larger amplitude. This is … interference (positive, productive, constructive).

2. When light strikes a transparent medium, some light is reflected and some is ... (transported, transmitted, carried through).

3. For angles greater than Qc, the light is reflected and none is refracted. This phenomenon is called … internal reflection (absolute, complete, total).

4. When light passes from air into a … medium as glass, the rays are refracted or “bent” toward the normal (more condensed, denser, more compressed).

5. A great deal of evidence suggests that light moves from the object to our eyes in straight-line … (paths, routes, tracks).

6. Refraction of light may cause optical … (deceptions, illusions, errors).

7. We know that … sound waves have wavelengths of centimeters to meters (perceptible, distinct, audible).

C. Fill in the correct word derived from the words in brackets