Light Oh no! I can’t see a thing. I think I lost my eye sight!!
Before we move on, We have… Representing the video clips available Representing the applets available Representing the websites available
Pupils should be able to
recall and use the terms for reflection, including normal, angle of incidence and angle of reflection.
state that, for reflection, the angle of incidence is equal to the angle of reflection and use this principle in constructions, measurements and calculations.
Introduction to Light Lesson objectives
Teacher demonstration In these demonstrations, your teacher will use a laser pointer to illustrate some interesting phenomena related to light. Pls turn to PB page 63 for detail. Lesson Trigger What is light? How do you know this is true? What evidence do you have to show this property?
1 Light travels in a straight line.
2 Light travels at a speed of 3 10 8 m/s.
3 Light can travel through vacuum.
Light is a wave that carries energy (light energy) from one place to another.
Properties of Light
Seeing the Light When light meets any surface, reflection of light occurs. Reflection always involves two rays – an incoming or incident ray that strikes a surface, and an outgoing or reflected ray that leaves the same surface. Reflection of Light incident ray reflected ray
Normal Laws of Reflection: This is an imaginary line perpendicular to the surface at the point where the light strikes. It is drawn to indicate the angle of incidence i and angle of reflection r . Reflection of Light normal
The incident ray, reflected ray and normal at the point of incidence all lie on the same plane.
What can you say about the angle of incidence and angle of reflection? What can you say about the orientation of the incident ray, reflected ray and the normal? Go to Exp11a.mpg i r incident ray reflected ray
angle of incidence = angle of reflection
i = r
Reflection is great! The fact that light reflects off objects allow us to see them. When light reflects from a smooth surface, it maintains its geometry. Incident parallel rays are reflected as parallel reflected rays. This is called specular reflection . Specular Reflection smooth surface
When a reflecting surface is rough, diffuse reflection occurs. The law of reflection still holds, but incident parallel rays do not reflect as parallel rays any more. In diffuse reflection, the reflected rays leave the surface at so many different directions such that the image is disrupted. Diffuse Reflection rough surface
On a mirror or a calm water surface, reflection is specular. The image formed on such surfaces are clear and sharp. However, if the surface is rough, or the water surface is disturbed, diffuse reflection occurs. The image formed is blur. Specular and diffuse Reflection
A directed straight line is normally drawn to represent a ray, which is the path taken by light from source to target. A beam of light is a stream of light rays, and is represented by a number of directed straight lines. Representing Light source target
Reflect on your daily encounters with light, (a) generate one other possible evidence to support the statement that 'light travels in a straight line', and discuss with your partner how you can demonstrate your statement with a safe, simple activity. (b) list two examples each for diffuse and specular reflections around you. Lesson Closure
We look at mirror everyday. What we see on the mirror is called our image. How do you describe your image on a plane mirror? Mirror mirror on the wall Go to Exp11b.mpg
Characteristics of image on a plane mirror
Same size as object
As far behind the mirror as the object is in front.
Virtual, as it cannot be captured on the screen
The image of an object is formed on a plane mirror when light ray from the object incidents on the mirror. object Light will reflect at a mirror surface such that the angle of incidence and reflection are equal. i r Image on a plane mirror plane mirror
There are many light rays reflected from an object to reach the mirror surface. However, only some light rays will be captured by the eyes. To view the image of an object in a mirror, the eye should be positioned along the direction where the reflected rays from the mirror can be captured by the eye. object Drawing Ray Diagrams plane mirror
To the eye, the light ray reaching the eye appears to come from the image behind the mirror. object This type of image is called a virtual image because it is formed at a place where there is no light from the object. The mirror simply makes the light appear to be coming from behind it. Drawing Ray Diagrams plane mirror
Steps involved in drawing ray diagrams 1 st - draw the image of the triangle, such that it is of the same size and same shape, and as far behind the mirror as the object is in front. Supposing a triangular object is placed in front of a mirror. We can draw a ray diagram to show how the eye sees the image in the mirror. Drawing Ray Diagrams plane mirror image object x cm x cm
2 nd – draw two diverging rays from any point on the image towards where the eye is positioned. 3rd – draw two diverging rays from the corresponding point on the object to the mirror to meet the reflected rays. Drawing Ray Diagrams Go to E-SimPhy_308.exe object plane mirror image
(i) Mark Caroline’s image on the glass window, at appropriate position. (ii) Draw ray diagrams to show how Sarah can see her friend by reflection on the shop window glass. (a) Sarah S went shopping with Caroline C one Sunday afternoon. Both girls stood in front of a shop admiring the window display. S C Shop window wall wall Ray diagram Practice 1 Where should you mark Caroline’s image on the glass window? Why? Which direction does the light ray seems to reach Sarah’s eye? Which direction is the light ray actually coming from?
Shop window wall wall C’ (b) Caroline moves away from Sarah to a new location C ’ to look at the display at the next shop. (i) Mark Caroline’s new image position (ii) Show by ray diagram, whether Sarah can still see her friend by reflection. (ii) Explain how your ray diagram helps you conclude on whether Caroline can be seen by reflection. Light rays from Caroline can still be reflected on the glass window, obeying the Laws of Reflection, where i = r . S C Ray diagram Practice 1 Which direction does the light ray seems to reach Sarah’s eye? Which direction is the light ray actually coming from? i r
Fun with Billiard
Visit the website below to practise your billiard skill! Have fun playing and learning the Physics of billiard, and see how the Laws of Reflection can be applied in playing billiard.
Beyond classroom activity
Enrichment / Extension Think about the various type of mirror you encounter in your daily life. (a) Look at the image formed. Can you explain the difference in the images formed from the one you use everyday in your bathroom? (b) Compare and contrast the type of surfaces of such mirrors, and explain why the images formed are different from one another? Lesson Closure
Pupils should be able to ◙ recall and use the terms for refraction, including normal, angle of incidence and angle of refraction. ◙ recall and apply the relationship, sin i sin r = constant to new situation or to solve related problems. ◙ define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium. Refraction of Light Lesson objectives
Behaviour of light When a beam of light encounters an obstacle in its path, a number of things can happen: 1 Reflection - 2 Refraction - 3 Absorption - The degree of each effect depends on the nature of the materials the light is incident upon. Refraction of Light
Using a laser pen, a beam of light is sent from water into air. When light strikes such transparent boundary, both reflection and refraction occur. Refraction of Light water What do you think will happen to its path of travel as it strikes the air-water boundary? reflection refraction laser pointer air glass reflection refraction refraction reflection
When will refraction occur? Why does refraction occur? 2 Refraction occurs because light travel with different speed when in media of different optical densities. Refraction of Light 1 Refraction occurs whenever light passes between transparent media of different optical densities.
Refraction of Light air glass The more optically dense the material, the slower the speed of light in that material The change in speed at the transparent boundary between two media causes light to change direction. Air has lower optical density – faster. Glass has higher optical density – slower. Air has lower optical density – faster. Optical density of glass > water > air > vacuum
Refraction Terminology Go to E-SimPhy_304.exe air glass air water i = angle of incidence r = angle of refraction r i i r incident ray refracted ray incident ray refracted ray
Light bends away from the normal when emerging from water into air, which is from an optically denser medium to an optically less dense medium. Light bends towards normal when entering into water from air, which is from an optically less dense medium to an optically denser medium. Refraction Rules bends away from the normal bends towards the normal Optical density of glass > water > air > vacuum
Summary air glass air glass i r from an optically less dense medium to an optically denser medium, i > r from an optically denser medium to an optically less denser medium, i < r bends towards the normal bends away from the normal i r
For each of the diagram below, complete the path of blue light as it emerges from the transparent medium. Ray diagram Practice 2
Refractive Index, n For example, the refractive index of glass at the air-glass boundary is given as: The refractive index of a medium is the ratio of the speed of light in one medium relative to the speed of light in the other medium. where v air and v glass are the speed of light in air and glass respectively. air glass v air v glass v air v air v glass n =
The ratio of the two sin s gives the same refractive index , and this relationship is called Snell’s Law . if 1 represents the angle of incidence in the less dense medium , and 2 represents the angle of refraction in the denser medium ; Refractive Index, n Go to Exp11c.mpg air glass 2 1 1 2 sin 1 sin 2 n = Then:
A ray of light approaches a glass-air boundary at an angle of incidence i = 30 . What is the refractive index of the glass if the angle of refraction r = 49 ? = 1.5 Sample Problem 1 angle in less dense medium angle in less dense medium Go to E-SimPhy_306.exe air glass 30 49 sin 1 sin 2 n = sin 49 sin 30 =
water air A ray of light approaches a water-air boundary at an angle of incident i = 35 . (i) Complete the path of the light as it crosses the water-air boundary. (ii) What is the angle of refraction if the refractive index of water n = 1.3? Sample Problem 2 35 sin 1 sin 2 n = sin 35 sin r = 1.3 = 26 r r
Prism disperses white light into its 7 component colours as the refractive index of each colour light is different in the glass prism. Analyse the diagram of dispersion, and infer the colour light thathas the greatest refractive index. Refraction of Light Lesson closure - Think about it
Pupils should be able to:
explain the terms critical angle and total internal reflection.
describe the action of a thin lens on a beam of light.
define the focal length for a converging lens.
draw ray diagrams to illustrate the formation of real and virtual images of an object by a thin converging lens.
Total Internal Reflection Lesson objectives
Refraction occurs as light passes across the boundary between two transparent media. Think – Pair share (5 min) Review previous lesson on refraction. Take turn to share with your partner facts about refraction. Jot done your discussion in your Physics notebook. Lesson Review air glass reflection refraction refraction reflection But why does light refract? What is the cause of such behaviour?
The picture below be easily reproduced with a laser pointer and a transparent semi-circle glass block in a darkened room. As light enters the glass block, it bends at the surface instead of traveling its original path. What do you think will happen if the angle of incidence in the glass block is increased gradually? Total Internal Reflection
A light ray from water is incident on the water - air boundary. The angle of incidence is gradually increased. (i) Calculate the angle of refraction if the refractive index of water is 1.3. (ii) What do you observe about the angle of refraction as the angle of incidence is gradually increased? air glass 40 o air glass 30 o 20 o air glass What is the maximum angle of refraction that can be produced? Total Internal Reflection As the angle of incidence increases, angle of refraction increases as well. 57 o 41 o 26 o
At a certain critical angle c , a maximum angle of refraction = 90 is produced. A weak reflected ray is also produced. If the next incident angle is greater than c , no more refraction will occur. The reflected ray becomes very strong and intense. 90 air glass strong reflection Total Internal Reflection Go to Exp11d.mpg c air glass i> c r
if incident angle > critical angle :
no light ray from the optically denser medium will be refracted.
all light rays will be totally internally reflected into the optically denser medium.
c weak reflection
Critical angle and Total Internal Reflection
Total internal reflection only occurs when :
a light ray is travelling from an optically denser medium to an optically less dense medium.
90 air glass strong reflection
the angle of incidence is greater than the critical angle.
Critical angle c is defined as the angle of incidence from a denser medium which produces an angle of refraction of 90 . c c weak reflection air glass i> c r
Sample Practice 1 For each combination of media, which light ray (A or B) will undergo total internal reflection if the incident angle is gradually increased? Explain your choice. air glass A B air water A B water glass A B In both cases, light is approaching the boundary from an optically denser medium to an optically less dense medium. A gradual increase in the incident angle will produce an increasingly larger angle of refraction.
A ray of light approaches a water-air boundary at a angle of incidence c , which causes it to undergo total internal reflection. Calculate the critical angle c if the refractive index of water n = 1.3 90 Sample Practice 2 sin 1 sin 2 n = sin 90 sin c = 1.3 = 50 c air water c
A laparoscope is a medical equipment inside a hollow, thin tube. It is connected to a camera and a high intensity light for doctor to see the structure inside our body.
Total Internal Reflection Enrichment Perform an internet search to find out how total internal reflection plays a part in different fields. Be prepared to share with your classmates what you have learnt from your research next week.
A ray of light that is incident at an angle on a transparent boundary will undergo refraction. Converging lens is a piece of glass which is thicker at the centre and thinner at the 2 ends. What do you think will happen to a light ray incident on its surface? Thin converging lens A converging lens air glass
A converging lens is simply a piece of glass which is thicker at the centre and thinner at the edge. Lens anatomy Optical centre - geometric centre of the lens Principal axis - This imaginary line divides the lens into equal upper and lower halves. Another imaginary line used to mark the mid-point of the lens
Parallel rays from distant object will all converge at a plane called focal plane. focal plane principal focus Lens anatomy focal length
Quick Check 1 What are the names of the various parts of a thin converging lens? Are you able to name each and every one of them? 1 2 4 3 5
Quick Check 2 What happens to each of the light rays as they pass through the converging lens? Can you complete the path they will take? F A beam of light parallel to the principle axis will bend at the principle focus F . A parallel beam of light not parallel to the principle axis will bend at some point on the focal plane. focal plane
Ray diagrams on Thin Converging Lens For simplicity, a thin converging lens is represented by a double arrowed line. F F F F
Rules for drawing Ray diagrams 2 Ray passing through optical centre will emerge unbent . 1 (a) Rays parallel to principal axis will meet at the principal focus. (b) Rays passing though principal focus will emerge from converging lens as parallel rays. F F
An object O is placed in front of a converging lens L. (i) Draw ray diagram to show how its image is formed. Label the image as I. (ii) Describe the image formed. F O The image formed is: inverted, diminished, real I Sample Practice 3 L
Think – Pair share (5 min) Review previous lesson. Take turn to share with your partner facts about converging lens. Jot down your discussion in your Physics notebook. Lesson Review A converging lens is simply a piece of glass which is thicker at the centre and thinner at the edge. What do I know? What have I learnt?
Magnifying glass Same size photocopier Camera Slide projector Spot light Trends and patterns on the images formed The type of the image produced by a converging lens are determined by the distance an object is placed away from the lens. Go to E-SimPhy_309.exe
Challenge yourself 1 The positions of an object and its image are as shown. (i) How do you determine the position of the lens? (ii) Where is the principle focus of the lens? F L
Challenge yourself 2 Object O A thin converging lens casts an image a distance away from the centre of the lens. Given the position of the principle focus as shown below, where should the object be positioned?
Challenge yourself 2 Object O Complete the path of the rays to show how an image can be formed by the converging lens. L F F Since both blue coloured rays originate from the tip of the object arrow, both rays should end at the tip of the image arrow.
Summary By the end of this lesson pupils should be able to: Use the terms normal, angle of incidence and angle of reflection (for reflection). State the laws of reflection and use it in calculations and measurements. Use the terms normal, angle of incidence and angle of refraction (for refraction). Solve problems using sin i ÷ sin r. Define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium. Explain the terms critical angle and total internal reflection.
Summary Describe the action of a thin converging lens on a beam of light. Define the term focal length for a thin converging lens. Draw ray diagrams to illustrate the formation of real and virtual images of an object by a thin converging lens.