LIGHT

PROPERTIES OF LIGHT Light always travels in straight lines. Light always travels at 2.98 x 10 8 ms -1 in air or a vacuum. ( kms -1 ) Light requires no medium (it can pass through a vacuum) White light contains all the colours If some colours are filtered out of white light you will see the remaining colours

ELECTROMAGNETIC WAVES This is a transverse wave where the oscillations are perpendicular (at right angles) to the direction the wave is moving.

LAWS OF REFLECTION Plane mirrors (plain ones!) Object = the thing you place in front of the mirror Image = what the mirror makes Location of the image = behind the mirror The image distance is the same as the object distance Size of the image = same as the size of the object The image is laterally inverted (flipped left to right) Nature of the image = virtual

LAWS OF REFLECTION i = angle of incidence r = angle of reflection Angle of incidence = angle of reflection

SPHERICAL MIRRORS REFER TO NOTES IN YOUR BOOKS

Ray Diagrams – Concave Mirrors LAW 1: Any light ray that passes through the focal point F, will be reflected back parallel to the principal axis.

Ray Diagrams – Concave Mirrors LAW 2: Any light ray that is parallel to the principal axis will be reflected through the focal point F.

Ray Diagrams – Concave Mirrors The image is focused at the point where the reflected light rays cross.

Ray Diagrams – Convex Mirrors LAW 1: Any light ray that is parallel to the principal axis is reflected back so that it is aligned with the FAR focus, F OBJECT

Ray Diagrams – Convex Mirrors LAW 2: Any light ray that is directed towards the FAR focus is reflected back parallel to the principal axis. OBJECT

Ray Diagrams – Convex Mirrors To locate the position of the image dot reflected rays back – the image is located where these rays cross. OBJECTIMAGE Reflected rays

Curved Mirror Formulas d 0 : The distance from the object to the mirror. d i : The distance from the image to the mirror. f: focal length F: focal point

Curved Mirror Formulas The image distance, d i can be negative or positive for a concave mirror. A positive value means the image is real. If d i is negative then it is a virtual image. For a convex mirror d i and f are always negative as convex mirrors form virtual images and have a virtual focus. For curved mirrors the image can be enlarged or diminished. The magnification factor m, is given by: H i = Image Height H o = Object Height

Example: NCEA 2005 QUESTION ONE Robbie and Amy are visiting the new aquatic centre in town. When Robbie is at the counter he looks at the security mirror on the wall. He notices that the mirror is curved, and it bends outwards in the middle. (a) On the diagram below, draw appropriate rays to show how his image is formed. (b)Calculate the magnification using your diagram. (c)Robbie moves so he is standing 3.0 m away from the pole of the mirror. The mirror’s radius of curvature is 2.0 m. Calculate the distance between Robbie and his image.

C = 2.0m, f = 2C, hence f = -1.0m d o = 3m d i = ? Answer: NCEA 2005 Rearrange and solve for d i. d i = 3/2 = -1.5 m f is negative as it is a convex mirror (virtual focus)

Lenses A simple lens is usually a piece of glass with spherical surfaces. A converging lens (convex) causes parallel light rays passing through it to converge to a point, F, the focal point. A diverging lens (concave) causes parallel light rays going through it to diverge (spread outwards). These rays spread out from the focal point, F. Converging lens (convex lens)Diverging lens (concave lens) F F

Ray Diagrams – Convex Lenses LAW 1: Any ray of light that is directed towards near focus F, is refracted parallel to the principal axis. Near FocusFar Focus

LAW 2: Any light ray that is parallel to the principal axis is refracted through the far focus F Ray Diagrams – Convex Lenses

LAW 3: Any light ray directed at the pole of the lens will go straight through. Ray Diagrams – Convex Lenses Images for convex lenses can be: Enlarged/Diminished, Upright/Inverted, Real or Virtual. Virtual SideReal Side Image Object

Ray Diagrams – Concave Lenses LAW 1: Any light ray that is parallel to the principal axis is refracted so that it is aligned with the near focus F Near FocusFar Focus

Ray Diagrams – Concave Lenses LAW 2: Any ray of light that is directed towards the pole of the lens will go straight through. Images for concave lenses are always virtual. Virtual Side Real Side

Formulae for Lenses Same as for mirrors except there are a few important points: For a convex lens: f is always positive. d i can be positive of negative. Positive means the image is real. Negative means the image is virtual. For a concave lens: f is always negative. d i is always negative. Concave lenses always for virtual images.

Example: NCEA 2003 QUESTION ONE: The Camera Moana is taking photos of her friend, Emma, using an ordinary film camera. A very simple form of a film camera consists of a light- tight box with a convex (converging) lens at one end and the film at the other end. (a) Complete the ray diagram below to show how the image of Emma is formed on the film. Drawn an arrow to represent the image.

(b) Describe the nature of the image formed on the film. Emma stands at a distance D cm in front of the camera lens. A sharp image of Emma is formed on the film. In this position, the distance of the lens from the film is 5.10 cm. The lens has a focal length of 5.00 cm. (c) Show that the distance from Emma to the camera lens, D, is 255 cm. (d) Calculate the magnification of the image. (e) The image of Emma on the film is 3.20 cm high. Calculate Emma’s actual height. Example: NCEA 2003

Excellence 2004 Mere can use a convex lens to produce a magnified image of an object on a screen. A possible set-up is shown in the following diagram, which is not drawn to scale. The convex lens has a focal length of 4.0 cm. A sharp magnified image is formed on the screen when the distance between the object and the screen is 25 cm. (a) Calculate the distance between the screen and the lens.

Refraction is the change in speed when a wave travels from one medium (substance) to another. When the wave changes speed through the medium the ray bends – this bending of light is called refraction. Refraction

The white light entering the raindrop is dispersed into the colours of the rainbow the same way a prism disperses light. The rain drops can be thought of as tiny prisms. The light is first refracted as it enters the surface of the raindrop, it then reflects of the back of the drop, and is again refracted as it leaves the drop. WHITE LIGHT RAIN DROP REFRACTED LIGHT

Example: Refraction occurs when light travels from air to water. Angle of incidence Angle of refraction Normal Incident Ray Refracted Ray Glass Air Water θiθi θrθr θrθr θiθi Normal Water Air Incident Ray Refracted Ray DRAWN AS

Refraction Normal: Line drawn in perpendicular to the medium boundary. Angle of incidence: Angle between the incident light ray and the normal. Angle of refraction: Angle between the refracted light ray and the normal. θrθr θiθi Normal Air Refracted Ray Water Incident Ray

Special Note: When light is travelling from an optically less dense medium to a optically more dense medium the refracted light ray will bend towards the normal. When light is travelling from an optically more dense medium to a optically less dense medium the refracted light ray will bend away from the normal.

Car example:

Refractive Index Refraction is caused by light changing speed. Different substances affect light speed by different amounts. Every substance has its own refractive index, n. (n can be thought of as the slowing down factor)

Refractive Index Absolute refractive index: n = speed of light in a vacuum = v o speed of light in medium v medium Relative refractive Index: n = speed of light in medium 1 = v 1 = n 2 speed of light in medium 2 v 2 n 1

Refractive Index Examples: Substancen Diamond2.42 Glass1.52 Perspex1.49 Water1.33 Air1.0 Vacuum1.0 Light slows down the most in _____________

Snell’s Law Snell’s Law: θ2θ2 θ1θ1 Normal n1n1 Refracted Ray n2n2 Incident Ray

Snell’s Law Snell’s Law: θ2θ2 θ1θ1 Normal n1n1 Refracted Ray n2n2 Incident Ray n 1 = refractive index of first medium n 2 = refractive index of second medium Ө 1 = angle of incidence Ө 1 = angle of refraction

Snell’s Law: Example NCEA 2004 QUESTION THREE: REFRACTION Lee is a keen astronomer. He discovers that good telescope lenses are often made of two types of glass of different refractive index cemented together. The diagram shows the path of a ray of light as it travels through two such pieces of glass. (a) Clearly mark the angle of incidence for the ray from flint glass to crown glass in the diagram. Label it  1. Use the information below to answer the questions (b), (c) Refractive index of crown glass= 1.52 Refractive index of flint glass = 1.66 Speed of light in crown glass = x 10 8 m s –1 Angle of incidence in flint glass = 19.8° (b) Show that the angle of refraction in the crown glass is 21.7°. (c) Calculate the speed of light in flint glass.

Refractive index of crown glass= 1.52 Refractive index of flint glass= 1.66 Speed of light in crown glass = x 10 8 m s –1 Angle of incidence in flint glass = 19.8° Working

Total Internal Reflection Remember: When light is travelling form an optically less dense medium to an optically more dense medium the light bends away from the normal. i.e. n 1 > n 2, Ө 2 > Ө 1 θ1θ1 θ2θ2 Normal n2n2 Optically dense Optically less dense n1n1 Weak Reflection

It is possible to get an angle of refraction equal to 90 o. When this happens the angle of incidence ( Ө 1 ) is called the critical angle ( Ө c ) Total Internal Reflection Angle of refraction Ө 2 = 90 0

When the angle of incidence ( Ө 1 ), is greater than the critical angle ( Ө c ) Total Internal Reflection occurs. All light reaching the boundary is reflected. Total Internal Reflection

Applications: Fibre Optics, Endoscopes.

(a) Calculate the size of the critical angle for the flint glass/crown glass boundary. (b) Give a detailed explanation of what is meant by the phrase ‘the critical angle for the flint glass/crown glass boundary’. Critical angle: Example NCEA 2004 Refractive index of crown glass = 1.52 Refractive index of flint glass = 1.66

Give a detailed explanation of what is meant by the phrase ‘the critical angle for the flint glass/crown glass boundary’. The critical angle, Ө c for the flint glass/crown glass boundary in the angle of incidence, Ө 1 that occurs when the angle of refraction, Ө 2 is equal to In this case the critical angle is ______. This is only possible when a light ray is traveling from an optically more dense medium (the flint glass) to an optically less dense medium (the crown glass). i.e. n 1 > n 2 For any angle of incidence greater than the critical angle, Total Internal Reflection will occur. When this occurs all light rays hitting the boundary will be reflected back. There will be no refraction, and therefore no light will pass into the crown glass. Note: To guarantee excellence for this question a diagram will always help support your answer! Critical angle: Example NCEA 2004

THE END Good Luck for the test!