Discover PHYSICS for GCE ‘O’ Level Science Unit 10 Light Reflection of Light

10.1 What is Light? Learning Outcomes In this section, you’ll be able to: Understand how light travels and how we are able to see.

10.1 What is Light? Light is a form of energy. Our eyes detect light in a range of colours – red, orange, yellow, green, blue, indigo and violet. Light travels at a speed of 3.0 × 108 m s-1 in vacuum.

10.1 What is Light? How does light travel? Light travels in straight lines.

10.1 What is Light? What are luminous and non-luminous objects? Luminous objects are objects which give off light of their own e.g. Lighted lamp, images on a TV screen Non-luminous objects are objects that do not give off light.

10.1 What is Light? How do we see? Figure 10.3 shows how our eyes see objects.

10.1 What is Light? Key Ideas Light is a form of energy. Light travels in straight lines called rays.

10.1 What is Light? Test Yourself 10.1 Figure 10.4 1. A student drew rays on the picture in Figure 10.4 to show how the eye sees the apple. Explain why the light rays are drawn incorrectly. Figure 10.4 Answer: The direction of light rays should be from the apple to the eye and not as shown in Fig 10.4. The light rays are reflected from the apple to the eye.

10.2 Reflection of Light Learning Outcomes In this section, you’ll be able to: Understand the terms used for reflection: Normal Angle of incidence Angle of reflection State that, for reflection, the angle of incidence is equal to the angle of reflection Apply the principle of reflection in constructing ray diagrams and performing calculations

10.2 Reflection of Light Laws of Reflection: 1st Law of Reflection The incident ray, the reflected ray and the normal to the reflecting surface all lie in the same plane. 2nd Law of Reflection The angle of incidence is equal to the angle of reflection  i =  r

10.2 Reflection of Light Table 10.1 Characteristics of regular and irregular reflection

10.2 Reflection of Light Characteristics of the image formed by a plane mirror: The image is of the same size as the object. It undergoes lateral inversion. It is upright. It is virtual. The distance of the image from the mirror is equal to the distance of the object from the mirror.

10.2 Reflection of Light Key Ideas The two laws of reflection are: The incident ray, reflected ray and the normal at a point of incidence all lie in the same plane. The angle of incidence is equal to the angle of reflection  i =  r The image formed by a plane mirror is: Same size as the object Laterally inverted Upright Virtual Distance of image from mirror is equal to the distance of object from the mirror

10.2 Reflection of Light Test Yourself 10.2 Draw a diagram to show clearly the incident ray, normal, reflected ray, angle of incidence and the angle of reflection. What is the relationship between angle of incidence and angle of reflection. Answer: i r incident ray reflected ray normal mirror The angle of incidence is equal to the angle of reflection.

10.2 Reflection of Light Test Yourself 10.2 What are the characteristics of an image formed in a plane mirror? Answer: The image is Upright Laterally inverted Same size as object Virtual Distance of image from mirror is equal to the distance of object from mirror

10.3 Constructing Ray Diagrams Learning Outcome In this section, you’ll be able to: Apply the principle of reflection in constructions, measurements and calculations.

10.3 Constructing Ray Diagrams Ray Diagrams for plane mirrors By drawing accurate ray diagrams, we can locate the position of a mirror image. Figure 10.15

10.3 Constructing Ray Diagrams Step 1: Image distance from mirror = object distance from mirror Measure accurately the perpendicular distance between object O and the mirror surface. Mark off the same distance behind the mirror to locate image I Fig 10.16

10.3 Constructing Ray Diagrams Step 2: Draw the light rays from the image to the eye Use dotted lines behind the mirror Use bold lines in front of the mirror surface. Fig 10.17

10.3 Constructing Ray Diagrams Step 3: Draw the incident rays from object O to the point of incidence on the mirror surface. Note that the angle of incidence = angle of reflection Fig 10.18

10.3 Constructing Ray Diagrams Drawing ray diagram for extended object By considering the extended object as a number of points, we apply the same 3 steps to locate the image. Figure 10.19

10.3 Constructing Ray Diagrams Multiple images in plane mirrors When two mirrors are placed 90o to each other, and an object O is placed between them, 3 images are formed. Figure 10.20

10.3 Applications of Mirrors Optical testing Figure 10.23 Plane mirrors are used to reduce the distance required for optical testing

10.3 Applications of Mirrors Mirrors at blind corners Such mirrors can be installed in shops to allow shopkeepers to watch out for shoplifters Can also be installed at blind corners to allow drivers to make a turn safely Figure 10.24 A blind corner mirror

10.3 Applications of Mirrors Instrument Scales By placing a mirror below the pointer of a scale, parallax error can be avoided when taking readings. This is done by ensuring that the image of the pointer is in line with the pointer itself. Figure 10.25 A voltmeter with a mirror below the scale

10.3 Applications of Mirrors Periscope Using two plane mirrors inclined at 45°, a periscope allows a person to look over a high wall or obstacles. Figure 10.26 A periscope

10.3 Applications of Mirrors Other uses Mirrors can be found in many optical instruments such as telescopes, overhead projectors and CD players.

10.3 Constructing Ray Diagrams Key Ideas Ray diagrams are used to locate the position of a mirror image. Some applications of mirrors include the periscope, for observing blind corners and avoiding parallax error on instrument scales.

10.3 Constructing Ray Diagrams Test Yourself 10.3 Figure 10.27 shows an extended object. Using ray diagrams, locate the image of the arrow. Describe the image formed. Answer:

10.3 Constructing Ray Diagrams Test Yourself 10.3 2. An optician’s eye chart is fixed 0.5 m behind the eyes of a patient looking into a mirror placed 3.0 m in front of him. Find the distance of the chart as seen by his eyes. Answer: The distance of the eye chart is 3.5 m from the mirror. Hence the image of the eye chart is 3.5 m in the mirror. Therefore chart’s image is 6.5 m away from the patient’s eyes. Image 3.0 m Chart mirror Patient’s eye H K E B N A 3.5 m 3.5 m