Light Form of Electromagnetic Energy Only part of Electromagnetic Spectrum that we can really see
Facts About Light The speed of light, c, is constant in a vacuum. Light can be: REFLECTED ABSORBED REFRACTED Light is an electromagnetic wave in that it has wave like properties which can be influenced by electric and magnetic fields.
Planck s Constant Energy of a photon is proportional to the frequency of the electromagnetic wave: E = hf h Planck s constant = 6.63x10-34 J-sec Light can act like a wave or like a particle. Classical electromagnetic wave theory explains light propagation and the effects of interference. Photoelectric effect and interaction of light with matter is better explained by assuming light acts as a particle.
Light Rays Light rays chart out a straight path. Unless the rays bump into something, they are always straight lines.
Reflection Specular reflection reflects all parallel rays at the same angle, like a mirror. Diffuse reflection reflects parallel rays at all different angles so that no image is transmitted, like a piece of paper. When viewing an object reflected by a mirror, the rays appear to originate from behind the mirror.
Reflection Law of Reflection - the angle of incidence (incoming ray) equals the angle of reflection (outgoing ray) The law works for FLAT, PLANE surfaces only. The angles are measured from a perpendicular line to the surface called a NORMAL.
Double-Reflection Example Two mirrors make an angle of 70 with each other. If a light ray is incident at an angle of 20, and reflects off both mirrors, what is the direction of the outgoing ray? The angle of reflection equals the angle of incidence from the first surface, or 20. The angle between the ray and the first surface is 90-20 = 70 ; the angle between the two mirror surfaces is 70. The interior angles of a triangle must sum to 180, the third angle (between the ray and the second surface) must be 180-70 - 70 = 40. The angle of incidence between the once reflected ray, and the second surface is 90-40 = 50. The angle of the outgoing ray is equal to the incident angle of 50.
Refraction of Light Refraction is the bending of a wave when it enters a medium where it's speed is different. As the speed of light is reduced in the slower medium, the wavelength is shortened proportionately. The frequency is unchanged; it is a characteristic of the source of the light and unaffected by medium changes. For any two given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. Snell s Law
Refraction of Light Refraction index n of a substance is a dimensionless number that describes how light, or any other radiation, propagates through that medium. Snell's Law relates the indices of refraction, n, of the two media to the directions of propagation in terms of the angles to the normal. The index of refraction is defined as the speed of light, c, in a vacuum divided by the speed of light in the medium, v. As the speed of light is reduced in the slower medium, the wavelength, λ, is shortened proportionately. The frequency, f, is unchanged; it is a characteristic of the source of the light and unaffected by medium changes.
Dispersion Index of refraction, n, is different for different wavelengths of light Any substance in which n varies with wavelength exhibits dispersion The angle of deviation, δ, is the angle at which the light is bent from it s original direction of travel
Total Internal Reflection When light is incident upon a medium of lesser index of refraction, the ray is bent away from the normal, so the exit angle is greater than the incident angle. Such reflection is commonly called "internal reflection". The exit angle will then approach 90 for some critical incident angle θ c, and for incident angles greater than the critical angle there will be total internal reflection.
Total Internal Reflection For any angle of incidence less than the critical angle, part of the incident light will be transmitted and part will be reflected.
Total Internal Reflection The critical angle can be calculated from Snell's law by setting the refraction angle equal to 90 For any angle of incidence less than the critical angle, part of the incident light will be transmitted and part will be reflected. The normal incidence reflection coefficient can be calculated from the indices of refraction. sin θ c = n 2 /n 1 n 1 n 2 Total internal refraction occurs only when light moves from a medium of high n to a medium of low n
Example 24.12 Light of wavelength 436mm in air enters a fishbowl filled with water then exits through the crown glass wall of the container. What is the wavelength of the light (a) while in the water and (b) while in the glass?
Example 24.18 A ray of light is incident on the surface of a block of clear ice at an angle of 40 with the normal. Part of the light is reflected and part is refracted. Find the angle between the reflected and refracted light?
Example 24.26 A submarine is 300m horizontally out from the shore and 100m beneath the surface of the water. A laser beam is sent from the sub such that it strikes the surface of the water at a point 210m from the shore. If the beam just strikes the top of a building standing directly at the water s edge, find the height of the building.
Example 24.26 A submarine is 300m horizontally out from the shore and 100m beneath the surface of the water. A laser beam is sent from the sub such that it strikes the surface of the water at a point 210m from the shore. If the beam just strikes the top of a building standing directly at the water s edge, find the height of the building.
Example 24.42 The light beam in the figure strikes surface 2 at the critical angle. Determine the angle of incidence, θ i. θ i
Example 24.42 The light beam in the figure strikes surface 2 at the critical angle. Determine the angle of incidence, θ i. θ i
Example 24.58 Two mirrors make an angle of 60 with each other. A ray is incident on mirror M 1 at an angle of 65 to the normal. Find the direction of the ray after it is reflected from mirror M 2. 60
Example 24.58 Two mirrors make an angle of 60 with each other. A ray is incident on mirror M 1 at an angle of 65 to the normal. Find the direction of the ray after it is reflected from mirror M 2. 60