# Reflection of Light

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## Plane Mirrors

-a plane mirror is a flat mirror that you see an exact image of yourself in

-when straight lines reflect from a flat reflector, the reflected waves are at the same angle to the reflector as the incident waves

-the perpendicular line to the mirror is called the normal

-the angle of incidence is the angle between the incident ray and the normal

-the angle of reflection is the angle between the reflected ray and the normal

FOR ANY LIGHT RAY REFLECTED BY A PLANE MIRROR:

ANGLE OF INCIDENCE=ANGLE OF REFLECTION

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## Virtual Images

-virtual image= image seen in a lens or a mirror, from which light rays appear to come after being refracted by the lens or reflected by the mirror

-virtual images CANNOT BE PROJECTED onto a screen     -virtual images are UPRIGHT

-the image formed on a plane mirror is virtual          -virtual images can be bigger or smaller than the object

-virtual images are normally formed by a diverging lens (when the object is closer to the lens than the princial focus)

-virtual images have an apparent left-right reversal. This is when you hold up your right hand in the mirror and it appears that you left hand has been raised. The image is LATERALLY INVERTED

-light rays never meet, only appear to because image is formed where light rays do not actually reach(image behind mirror)-in plane mirrors the object and the image are an equal distance from the normal and there is amaginfication of 1 (if the object is a meter, it will appear to be a meter in the mirror)

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## Real Images

-can be projected onto a screen

-the image formed when the light rays are focussed onto a screen

-smaller than the object

-inverted

-formed by a convex lens

-formed when the object is at or further from the principal focal point

-the light rays cross

Examples:cinema screen, retina, camera

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## Refraction

-refraction is a property of all waves

-light waves are refracted when they travel across a boundary between air and a transparent medium or between two transparent medium. Speed of light changes at such a boundary

-Light travels towards the normal when it travels from air to glass and away from the normal when it travels from glass to air and these changes occur because light travels more slowly in glass than air. We say that glass is ''optically more dense" than air

-When light enters a more dense medium, it is refracted towards the normal. Angle or (R) is smaller than the angle of (I)

-When light enters a less dense medium, it is refracted away from the normal Angle of (R) is greater than (I)

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## Refraction in a prism

-when a narrow beam of white light passes through a triangular glass prism, the ray comes out in a different direction to the incident ray and is split into the colours of the spectrum

-white light contains all the colours of the spectrum

-each colour of light is refracted slightly differently

-the splitting of white light into the colours of the spectrum is called DISPERSION

-a rainbow is caused by the refraction of light when sunlight shines onto the ran droplets and the droplets refract the sunlight into the different colours of the spectrum

-the glass slows down some colours of light more than others

-the part of the wave that enters the glass first gets slowed down before the rest, which changes the angle of the whole wave.

-violet light gets slowed down more than red light does

-the different colours have different wavelenghts. Red has the longest wavelength and violet has the shortest wavelength. A shorter wavelength means that the light waves are interacting more with electrons and this slows them down more.

-the band of colours formed due to the dispersion of light is called the SPECTRUM

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## Refractive Index and Snell's Law

-when light travels from air to a transparent medium, the angle of refraction depends on the medium as well as the angle of incidence

-For light travelling from air to glass: 1) angle of refraction is always less than the angle of incidence   2)the greater the angle of incidence, the greater the angle of refraction

-Snell's Law showed that sin(i)÷sin(r) always has the same value for the same medium

-REFRACTIVE INDEX (N)= SPEED OF LIGHT IN A VACUUM (AIR)                                                                                                                       SPEED OF LIGHT IN THE MEDIUM                                                                                                                        -REFRACTIVE INDEX OF THE MEDIUM, N= sin(i)                                                                                                                                                                                                    .

-FAST is a way to remeber how rays bend towards the normal                                                                                           -When a light ray travels from a transparent medium into air at a non-zero angle of incidence the light ray is refracted AWAY from the normal and the larger the angle of incidence, the larger the angle of refraction

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## Optic Fibres and Endoscopes

-optical fibres are very thin glass fibres designed to transmit light of infrared radiation and the glass absorbs little light

-light goes in at one end and undergoes repeated total internal reflection, which means even when the fibre is bent, light will emerge at the other end

-the endoscope is used by surgeons to look inside the body without having to cut it open, for example, you can look inside a patients stomach by insterting an optic fibre through the throat

-the endoscope contains two bundles of optical fibres alongside each other. One is used to shine light into the cavity. The other is used to see the internal surfaces of the cavity and has a samll lens over it that can be displayed on a digital camera at the other end.                                                                                                                                                                    -laser light could be used as a source of energy in an endoscope to carry out surgical procedures. It can cut away/burn away and destroy diseased tissue and can also be used to cauterise leaking blood vessels. This is possible with laser light because energy can be focussed in a very small area. The colour of the laser light can be matched to the type of tissue.

-choosing the appropriate colour ensures the most effective absorption                                                                                -eye surgery on the retina can be carried out by applying a laser beam through the pupil of the eye for a very short time

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## Total Internal Reflection

-when light is refracted from glass to air, there is also a partial line of reflection and this angle of reflection is equal to the angle of incidence

-as the angle of incidence increases, so does the angle of refraction

-when the angle of incidence is increased beyond a critical angle (the (i)angle where the ray emerges along the boundary and is at 90 degrees), the light ray is totally internally reflected

-when total internal reflection occurs, the angle of reflection is equal to the angle of incidence

-REFRACTIVE INDEX=1÷ sin(c)                 sin(c)= 1÷n        c=critical angle

-TIR only occurs when light is trying to enter a less dense medium

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