What is Light? Light is a form of electromagnetic radiation that can be seen by the eye. What is Electromagnetic Radiation? Electromagnetic radiation is a term used to describe waves that are created by the motion of electrically charged particles. All forms of electromagnetic radiation travel at the same speed: 300 000 000 metres/second (3x10 8 m/s or 300 000 km/s). There are many different forms of electromagnetic waves including: Figure 1: Different types of Electromagnetic Radiation xrays ultraviolet Violet light Red light microwave radio wavelength Electromagnetic radiation can be classified according to the wavelength of the waves. The wavelength of radio waves used by radio stations can be several hundred metres long, or as short as a few metres. The wavelength of microwaves in microwave ovens is about 12 cm. From the diagram, has the shortest wavelength. Furthermore, different colours of light have different wavelengths. Red has a wavelength than violet light. Our eyes cannot see electromagnetic radiation that has a longer wavelength than red (ie microwaves) and a shorter wavelength than violet (ie xrays). Vision Our eyes can see objects that either emit light or reflect light
a) Emission There are many sources of light. The sun is the earth s primary source of light, however many other things can also emit light. Light is emitted from light bulbs, televisions and computer monitors, glow sticks, fireflies, LEDs, fireworks and during stormy weather (lightning). Match the following terms to the definitions: chemiluminescence, bioluminescence, incandescence, fluorescence, phosphorescence, triboluminescence. Term Definition Examples the emission of light by a living organism through a chemical reaction the emission of light by a material after being exposed to ultraviolet radiation or other types of radiation the emission of light through a chemical reaction the emission of light by a material that undergoes friction and/or the emission of light that results from the breaking of certain crystals the emission of light by a heated solid the emission of light by a material while exposed to ultraviolet radiation b) Reflection The paper you are currently reading, or the pencil/pen you write with do not emit light. When these objects are placed in a dark room, they cannot be seen. Most objects that we use on a daily basis can only be seen because they reflect light, which then enters the eye. If the light reflected by an object does not enter your eye, then the object cannot be seen. Properties of Light Light travels in straight lines. To represent light, a ray is drawn that illustrates the direction light travels in. light ray
Reflection of Light When a flashlight is aimed at a shiny smooth surface, such as a mirror, the light will reflect. The four rays (figure 1) represent the light coming from the flashlight. These are called rays. Figure 1: Label the mirror, incident rays and reflected rays When light shines on a surface, we say that light is on the surface. The four rays that are lighter represent the light that is, and are called rays. Reflection of Light - Geometry Angles in optics are measured with respect to the. The is an imaginary line that is drawn to the surface. Figure 2: Label the normal, the incident ray, the reflected ray, the angle of incidence and the angle of reflection. Measure the angle of incidence The angle of is measured between the normal and the incident ray. The angle of is measured between the normal and the reflected ray. Exercise 1: Measure the angle of incidence and draw the reflected ray. Label the normal. a) b) c) d) e) f) g) h)
Exercise 2: Draw the normal for the following figure at points a b c d and e. e d a b c Refraction of Light Light sometimes can pass through a medium or substance that it is incident on. For example, in figure 3 light initially is incident on the glass and passes through it. Light not only reflects off of the glass but also can travel through the glass. The ray of light that travels through the glass (and then through the water) is called the ray. Notice that the pencil appears bent. This occurs because light rays change direction when traveling through a different medium or substance (from air to glass to water). For figure 4, light travels from air into glass. Light travels in straight lines, but changes direction (refracts) when entering the glass. Figure 3: Figure 4: Label the normal, incident ray and refracted ray. Draw the reflected ray. Measure the angles of incidence and refraction. air glass Exercise 3: Label the normal and draw the reflected ray. Measure the angles of incidence and refraction. a) b)
Exercise 4: Light travels from air into glass. The angle of incidence is 40 degrees and the angle of refraction is 29 degrees. Draw the incident ray, the refracted ray, the reflected ray and the normal. The angle light refracts at when it enters another medium (substance) also depends on the of the medium. Different mediums have different optical densities. The more optically dense a medium is, the more light or bends. In the example below, glass is more than water and hence causes light to bend more. For the water, draw a refracted ray with an angle of refraction of 34 degrees. For the glass, draw a refracted ray with an angle of refraction of 28 degrees. Measure the angle of incidence and label it in the figure. water glass Exercise 5: The two pieces of glass are similar, however one piece has a higher optical density. Draw the reflected rays. The angles of incidence are the same. Which piece of glass is more optically dense? A B
Refraction of Light and Index of Refraction The index of refraction is a quantity that is used to measure the. As the index of refraction for a medium increases, the optical density. The index of refraction is defined by the following ratio speed of light in a vacuum Index of refraction of a medium = speed of light in the medium n = c v The speed of light in a vacuum is m/s. This speed is given the special symbol c c = 3x10 8 m/s As light travels from the sun through the vacuum of space, it travels at the speed of light. The index of refraction for a vacuum is 1 (n = 1). As sunlight enters the atmosphere, it very slightly, but the index of refraction for air is approximately equal to 1 (n = 1). As sunlight enters water, it slows down significantly. The index of refraction for water is 1.33 (n = 1.33). For other substances, such as or, light slows down by an even greater factor (n = 1.5 and n = 2.42 respectively). Example a: Calculate the speed of light in diamond if its index of refraction is 2.42. Example b: Calculate the index of refraction if the speed of light in a material is 1.5 x 10 8 m/s. For diamond, an index of refraction of 2.42 means that light travels 2.42 times slower in diamond than in space. As light travels from air into this particular material, the speed of light decreases by a factor of. Refraction of Light and Index of Refraction: Case 1 Light travels from a medium with low optical density into a medium with high optical density In this example, light travels from air (n = 1) into glass (n = 1.6). Light travels into a medium that is more optically dense (from a low value for n to a higher value for n). In general, when light travels into a medium that is more optically dense, the angle of incidence is always greater than the angle of refraction. For these situations, we say that light "bends" the normal. Figure 5: Draw the normal and the reflected ray. Measure the angle of incidence and the angle of refraction. Glass, n = 1.6
How would the figure change if the index of refraction for glass were 2.2? Draw the refracted ray. Glass, n = For which angle of incidence does light not change directions (does not bend)? Refraction of Light and Index of Refraction: Case 2 Light travels from a medium with high optical density into a medium with low optical density In this example, light travels from glass (n = 1.4, a medium with high optical density) into air (n = 1, a medium that is less optically dense). When light travels into a medium that is less optically dense (from a high value for n to a lower value for n), the angle of incidence is always less than the angle of refraction. For these situations, we say that light "bends" from the normal. An everyday example of this situation is found in Figure 6: Draw the normal. Measure the angle of incidence. Measure the angle of refraction. Label the normal, incident ray and refracted ray. Refraction of Light and Index of Refraction: Case 2 - The Critical Angle Sometimes if light travels into a medium with a lower index of refraction (less optically dense), there exists an incident angle for which light does not refract. The smallest incident angle for which light does not refract is called the. If the incident angle is greater than the critical angle, light is 100% reflected. This is called.
Total internal reflection occurs for angles of incidence that are greater than the critical angle for the mediums. The critical angle depends on the optical density of the mediums. Figure 7: Light incident at an incident angle of 46, the critical angle. Exercise 7: Draw the normal for each example and measure the angles of incidence. a b c Glass, n = 1.6 Glass, n = 1.6 Glass, n = 1.6 angle of incidence < critical angle angle of incidence = critical angle angle of incidence > citical angle total internal reflection total internal reflection For glass that has an optical density of 1.6, the critical angle is. Exercise 8: As light travels from a mysterious substance (n is about 1.31) into air (n = 1), the critical angle is about 50 degrees. Which of the following diagrams is not possible based on the critical angle being 50 degrees? (for each question, select A or B and write a brief statement explaining your choice) 1. A B 78º 78 78 78
2. A B 22 22 22 22 3. A B 22 22 22 22 The Semi-Circle Glass Figure 8: Draw the reflected ray. Measure the angle of incidence and the angle of refraction. Why does light not bend at point qq as it enters the glass? qq q For glass that has an optical density of 1.5, the critical angle is. Optical Density, Index of Refraction and Air Air has an index of refraction that is approximately 1. However, the index of refraction of air varies with temperature, hence different temperatures of air have different optical densities. This can be seen on a hot summer day. If you view the hood of a car or pavement, it appears that the air above it shimmers. Due to convection, air of different temperatures flows above the pavement or the hood, constantly changing the optical density. Hence, light no longer travels in a straight line and refracts. The refraction ultimately causes shimmering to occur.