n has to be a positive whole number (n N) means that n 3. sin u ◊ 1Qnl ◊ dQn ◊ d
500 103 2.0 106 m l Qn ◊ 2.0 106
5.5 107 3.636
EXERCISE 14.2 WAVELENGTH OF LIGHT
Q1 A laser is shone through a diffraction grating with 700 lines per millimetre. The angle between the zero and first order images is 30.What is the wavelength of the light source?
Q2 A monochromatic light source of wavelength of 420 nm is used to demonstrate the wave nature of light, using a diffraction grating with a constant of 2.0 106 m. What is the angle opened up to the (i) first, (ii) second and (iii) third order images?
Q3 A laser is shone through a diffraction grating with 500 lines per millimetre. A series of lines are seen on a screen 2 m in front of the grating. The distance between the central line and the nearest line on the right is 72 cm. What is the wavelength of the light from the laser?
Q4 What is the maximum number of bright fringes when a monochromatic light source, of wavelength 650 nm, is shone through a grating with a spacing of 2.2 106 m? (Remember there are images to the left and the right, as well as the central image.)
The Electromagnetic Spectrum
Light is made up of electromagnetic waves,EM waves (also called electromagnetic radiation), that travel at a speed of 3 108 m s1.
SPECTRUM The spectrum is the range of wavelengths present in the electromagnetic radiation. λ
Magnetic field Electric field
Electric and magnetic fields vibrate at right angles to each other, and also to the direction propagation of the wave. EM waves are therefore transverse waves. At Leaving Cert. level we tend to ignore the magnetic field as its position is determined by the electric field.
EM waves are usually drawn as a single sine wave.They are not made of matter (unlike mechanical waves) and can travel through a vacuum.
Physicists use the term ‘light’ to describe all wavelengths of electro- magnetic radiation (Fig 14.11).However, it is only a very narrow range of wavelengths that form visible light. Visible light has a range of
