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ClassAct SRS enabled. • In this presentation you will: • explore the reflection and refraction of light rays

Light travels in straight lines called rays. Light rays behave in different ways when they encounter objects. They might pass through an object, They might become absorbed or partially absorbed by objects, They might be reflected by non-transparent surfaces. In this presentation you will explore what happens to light rays when they hit a mirror or travel through a transparent solid. You will then see how images appear when light rays travel through lenses. Next >

Reflections Objects are seen by the eye either because they reflect light or because they emit light. We see stars (including the Sun!) because they emit light. We see the Moon because it reflects light from the Sun. Next >

Reflections A mirror will reflect rays of light. Reflected ray Normal Incident ray A light ray that hits a mirror is calledthe ray of incidence (or incident ray). r i The light ray that bounces off the mirror is called the ray of reflection (or reflected ray). The normal is a line perpendicular to the mirror. The angle of incidence from the normal is represented by i. The angle of reflection from the normal is represented by r. Next >

The Law of Reflection Reflected light obeys the law of reflection, “theangle of reflection equals the angle of incidence”. Parallel light rays hitting a very smooth reflective surface will all be reflected through the same angle. This is called specular reflection. Next >

The Law of Reflection The reflection of light from rough surfaces is called diffuse reflection. Such surfaces consist of a large number of different reflecting planes, each one producing specular reflection. Next >

Question 1 Which light ray is called the incident ray? A) The light ray that hits the mirror. B) The light ray that bounces off the mirror. C) The line perpendicular to the face of the mirror.

Question 2 For reflection, the angle of incidence is: A) at 90° to the face of the mirror. B) the same as the angle of reflection. C) twice the angle of reflection.

Normal Reflected ray Incident ray Air Glass Refracted ray Reflection and Refraction When a ray of light (incident ray) travels from one transparent material or medium to another, part of the ray is reflected (reflectedray) at its boundary and part enters the second piece of material. The ray entering the second piece of material is known as the refractedray. The change in direction of the ray of light, entering the second piece of material, is due to light changing speed. This is known as refraction. Next >

Water (lower speed) Air (higher speed) Direction ofmovement Wavelength has increaseddue to increased speed boundary Refraction For example, light travels slower through water than through air. Therefore, when light travels from water into air, it speeds up. This is shown in the diagram above. Each line represents a wave front moving from one medium to another. If the light is traveling at right angles to the boundary, it passes through the boundary with no deviation. Next >

New direction Air Water Refraction However, when the light is not traveling at right angles to the boundary, the direction of the light changes. To explain this, it is necessary to again think of light as a wave front, moving from one medium to another. As the first part of the wave enters the boundary, it speeds up. The rest of the wave continues at the same speed. The result is that the wave front progresses further on one side than the other, causing the wave front to swerve. Next >

Refraction in a Glass Block A simple experiment can be performed to see refraction. This involves passing a ray of light at an angle into a glass block. When the ray, traveling in air, enters the glass, it slows down. This causes the ray to bend toward the normal. The light then returns to its original speed when it leaves the glass. This causes the ray to bend away from the normal. Next >

c n = v Refractive Index The refractive index is defined as the ratio of the velocity of light in a vacuum to the velocity of light in the medium: where n = refractive index c = velocity of light in vacuum in m/s v = velocity of light in the medium in m/s The refractive index of a material has no units as it is a ratio. Note that it is always larger than 1, as the velocity of light in a medium can never exceed the velocity of light in a vacuum. Next >

Substance Refractive Index Diamond 2.419 Glass (Crown) 1.52 Ice 1.309 Water 1.333 Air @ 0°C, 1atm 1.000293 Refractive Index The table shows some common substances and their refractive indices. The lower the refractive index, the higher the speed of the light traveling through the material. Next >

Willebrord Snell (1580-1626)discovered the relationship between the angle of incidence (i) and the angle of refraction (r) of light passing through two media. Normal Angle of incidence i REFRACTIVE INDEX n1 REFRACTIVE INDEX n2 r n1 sin i = n2 sin r Angle of refraction Snell’s Law of Refraction The equation relating these angles is given as: Where n1 and n2are the refractive indices for two different media. Next >

Normal a Air b d Water c Question 3 Which of the following fractions gives the ratio of the refractive index of water to the refractive index of air? A) sin a / sin c B) sin a / sin d C) sin b / sin c D) sin b / sin d

Normal Incidentray i r Refracted ray Incidentray ic n1 refractive index of air Refracted ray sin ic= = n2 refractive index of glass Critical Angle As you have seen, a light ray will refract when passing at an angle through a boundary between two materials. However, if the incident ray comes from a more optically dense material to a less dense material (for example glass to air) at a critical angle, ic, then the light will refract at 90° to the normal, along the boundary between the two materials. The critical angle ic is given by: Note: this is only true when n2 > n1 Next >

Incidentray Refracted ray i Total Internal Reflection If the incident ray exceeds the critical angle, then all of the light will be internally reflected back from the boundary region into the first material. The boundary region acts as a mirror. This is calledTotal Internal Reflection. An interesting application where total internal reflection is used, is in fiber optics. Next >

Question 4 If the critical angle between glass and air is 41°, what would happen to a light ray, traveling in glass, that hits the glass-air boundary at an angle of incidence of 50°? A) Some of the light would be refracted. B) The light would come out along the glass-air boundary. C) All the light would be internally reflected.

Dispersion Sunlight is often called white light, since it is a combination of all the visible colors. Since the refractive index of a transparent material, such as glass, is different for each color, the angle of refraction will depend on the color of the light ray entering the material. If a piece of glass has parallel sides, the light will return in the same direction that it entered the material. Next >

Dispersion If the material is shaped like a prism, the angles for each color will be exaggerated, and the colors will be displayed as a spectrum of light. This breaking up of white light into its constituent colors is called dispersion. Next >

Converging Lens A converging or convex lens is thicker in the middle than at the edges. Focal point A converging lens refracts light rays passing through it, parallel to the principal axis, to a point called the focus or focal point. Focal length Principal axis The distance of the focal point from the center of the lens is called the focal length of the lens. Next >

Converging Lens The fact that a convex lens is thicker across its middle is an indicator that it will converge rays of light which travel parallel to its principal axis. Any light emitted from the focal point is refracted by the lens to become parallel to the principal axis. Next >

Focal length Principal axis Focal point Principal point Diverging Lens A diverging or concave lens is a lens that is thinner in the middle than at the edges, causing rays traveling parallel to its principal axis to diverge. The focal point of a diverging lens is found by extending the refracted rays backwards behind the lens, to a point where the refracted rays intersect. A diverging lens is said to have a negative focal length. As light doesn’t actually come from this point it is called a virtual focus. Next >

Question 5 Which of the diagrams shows the correct ray paths for a light beam passing through a convex lens? A) B) C) D)

Summary After completing this presentation you should be able to: • show knowledge and understanding of the reflection of light rays. • show knowledge and understanding of the refraction of light rays. • show knowledge and understanding of how lenses use refraction. End >

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