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Light

Light. Reflection of light. When light is reflected off of a mirror it forms an image. Normal. Incident ray. Reflected ray. Mirror. Reflection of light. The law of reflection states that: the angle of incidence = the angle of reflection i = r. Normal. Incident ray. Reflected ray. i.

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Light

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  1. Light

  2. Reflection of light • When light is reflected off of a mirror it forms an image. Normal Incident ray Reflected ray Mirror

  3. Reflection of light • The law of reflection states that: the angle of incidence = the angle of reflection i = r Normal Incident ray Reflected ray i r Mirror

  4. Properties of reflection • An image is formed when light is reflected. • The image is the same size as the object and the same distance behind the mirror as the object is in front.

  5. Properties of reflection • The image is laterally inverted (left is switched with right) • The image is virtual (it’s not real)

  6. Summary • The normal at a point on a mirror is perpendicular to the mirror. • The image of an object in a plane mirror is virtual, the same size, laterally inverted, and the same distance from the mirror. • For a light ray reflected by a mirror: the angle of incidence = the angle of reflection • Homework: pg. 117 #1,2

  7. Refraction of light

  8. Refraction of light • Refraction occurs when light pass from one transparentmedium (substance) to another. For example it happens when light passes from air to glass. Air Glass

  9. Changes • Refraction occurs because the speed of the light changes in different mediums. • When it travels from a fast medium to a slow medium, the light bends towards the normal. Air Glass

  10. Changes • When light travels from a slow medium to a fast medium, the light bends away from the normal. Glass Air

  11. Apparent Depth • Why do things look different when they are under water?

  12. Apparent Depth • Because light waves are refracted when they change medium, they make object appear differently. They create an image of an object that is not exactly where the object is.

  13. Astronomical Refraction • Light traveling from stars is refracted when it crosses into the Earth’s atmosphere. A vacuum is a fast medium and air is the slow medium, so the light bends towards the normal.

  14. Summary • Refraction is the change of direction of light when it travels from one transparent medium to another. • Refraction is towards the normal when it goes from a fast to slow medium. • Refraction is away from the normal when it goes from a slow to fast medium. • Classwork: pg. 119 #1,2

  15. Refraction index

  16. Refraction • The change of direction of a light ray depends on which substance to light is traveling in. • Light well bend differently when traveling through water than it would through glass.

  17. The refractive index • The refractive index of a substance is n. the speed of light in air n = the speed of light in the substance

  18. Example • The speed of light is air is 300,000 km/s. The speed of light in glass is 200,000 km/s. Calculate the refractive index. the speed of light in air n = the speed of light in the substance 300,000 km/s n = 200,000 km/s n = 1.50

  19. The refractive index • It is a measure of how much a substance changes the direction of light. • The refractive index for water is 1.33 and glass is 1.50. Therefore, glass will change light more than water.

  20. The refractive index • How could we find the refractive index if we do not know the speed? A man named Snell figured out how in 1618. • Snell’s law: n – the refractive index i – the angle of the incident ray from the normal r – the angle of the refracted ray from the normal sin i n = sin r

  21. The refractive index • Remember that the angle of incidence and angle of refraction are measured from the normal. sin i n = sin r i Air r Glass

  22. Example • A light ray travels through air into water. The angle of incidence is 50o and the angle of refraction is 35o. Calculate the refractive index. sin i n = sin r sin 50 n = sin 35 1.34 n =

  23. Summary • The refractive index of a substance is • For a light ray traveling from air into a transparent substance • Homework: pg. 121 #1,2 the speed of light in air n = the speed of light in the substance sin i n = sin r

  24. Total internal reflection

  25. Refraction of Light • When light crosses a boundary from a slow medium to a fast medium, the light moves away from the normal. Glass Air

  26. Refraction of Light • The larger the angle of incidence the larger the refracted angle. Glass Air

  27. Refraction of Light • There is an angle where the angle of refraction will be 90o. • This is called the critical angle. Glass c Air

  28. Refraction of Light • If the angle of incidence is larger than the critical angle, there will be no refracted ray, only a reflected ray. • This is called total internal reflection. Glass Air

  29. Optical Fibres • Optical fibres use total internal reflection to transfer information. The light travels down the wire but does not escape because it is reflected from the boundary.

  30. Summary • When a light ray travels from a slow medium into a fast medium there is an angle where the refraction is 90o. This is called the critical angle. • When the incident angle is less than the critical angle, most of the light is refracted. • When the incident angle is at the critical angle the light is refracted along the surface. • When the incident angle is greater than the critical angle, all of the light is reflected. This is called total internal reflection. • Homework pg. 123 #1,2

  31. The converging Lens

  32. Converging lens • A converging lens changes the direction of light passing through it so it is directed to one point called the focal point. • The following objects use converging lens:

  33. Converging Lens • The following is when rays travel parallel to the principal axis of the lens.

  34. Ray Diagrams • When completing ray diagrams there are three important rays to consider. • The ray traveling parallel to the principal axis is refracted through the focal point. • The ray traveling through the center of the lens is not refracted. • The ray traveling through the focal point is refracted parallel to the principal axis

  35. Applications of Lens • The camera lens is used to make a smaller, inverted, real image on a film. • The distance between the lens and the sensor must be changed depending on the distance of the object.

  36. Applications of Lens • A projector is used to give a larger, inverted, real image.

  37. The Magnifying Glass • The magnifying glass is used to create an image that is virtual, upright and magnified.

  38. Summary • The principal focus of a converging lens is the point where parallel rays directed straight at the lens converge. • The focal length of a converging lens is the distance from the lens to the principal focus of the lens. • A real image is formed by a converging lens if the object is further away than the principal focus of the lens. • Copy the table at the bottom of page 127 into your notebook. • Homework pg. 125 #1,2 and pg. 127 #1,2

  39. Electromagnetic Waves

  40. Review • The index of refraction is: • The wave equation is: v = fλ the speed of light in air n = the speed of light in the substance

  41. Definition: Spectrum • A spectrum is a way of comparing objects by ordering them. The following is an “economic spectrum”: Poor Middle Rich • We can use spectra to compare things instead of using charts or graphs.

  42. The Light Spectrum • Light from a lamp or the Sun is called white light. It contains all of the colours on the visible spectrum. • Light that is only one colour is called monochromatic. • The following is a picture of all the colours: “Roy G Biv”

  43. Different colours • Different colours have a different wavelength (λ) and a different frequency (f). • Red has a longer wavelength and a lower frequency. • Violet has a shorter wavelength and a higher frequency.

  44. Dispersion • The different colours of light travel at different speeds in glass. • If white light is passed through a glass prism, the colours will be refracted differently. • This is called dispersion.

  45. The Electromagnetic Spectrum • The visible light spectrum (what we can see) is a small part of a larger spectrum called the electromagnetic spectrum. • Electromagnetic waves (EM waves) are disturbances that transfer energy from one place to another. • The following are some examples of EM waves: radio waves, microwaves, infra-red, ultraviolet, x-rays

  46. The Electromagnetic Spectrum

  47. Properties of EM Waves • EM waves can have different frequencies and different wavelengths. • All EM waves have a speed of 300,000,000 m/s in a vacuum.

  48. Radio Waves • Wavelength ≈ 1km • Uses: communications (radio, TV, mobile phones)

  49. Microwaves • Wavelength ≈ 1m • Uses: communications (satellites), heating food

  50. Infra-red Radiation • Wavelength ≈ 1mm • Uses: cookers and heaters, cameras, optical fibre, TV remote controls.

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