SPM Phyiscs - Light
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SPM Phyiscs - Light

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Simplified notes for SPM students

Simplified notes for SPM students

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SPM Phyiscs - Light SPM Phyiscs - Light Presentation Transcript

  • Light and Optics Form 4 Physics (SPM) – Chapter 5
  • Light   Electromagnetic radiation that is perceived by the human eye Has two forms     Waveform Particleform Light travels in the form of rays in a straight line Light rays tend to spread away from the source
  •  Wave properties of light        Reflection Refraction Diffraction Interference Light is a form of energy Visible light is made up of 7 colours of various intensities Light, like any electromagnetic radiation travels through vacuum at a speed of approximately 3X108 ms-1
  • Optics   Study of interaction of light with objects that reflect or scatter light Light is able to    Completely penetrate transparent objects Partially penetrate translucent objects Not penetrate opaque objects
  • Light rays
  • Key notes          Object – material that emits light rays Image – representation of object after light has reflected or refracted Ray diagram – diagram showing path of light through reflection or refraction Line of sight – order or position of the eye to view the image formed Solid lines – real light rays Dotted lines – virtual light rays Convergence – meeting of light rays at a point Divergence – spreading of light rays from a point Focus point – point where light rays are converged or diverged
  • Reflection       Occurs when the direction of light changes when it strikes an opaque material Light ray before reflection – incident ray Light ray after reflection – reflected ray Imaginary line that is perpendicular to surface – normal line Angle of incidence, i – angle between incident ray and normal line Angled of reflection, r – angle between reflected ray and normal line
  • Normal Incident ray i r Reflected ray Plane
  •  Two types of reflection  Specular reflection   Occurs when light strikes a smooth and shiny surface Diffused reflection  Occurs when light strikes an uneven surface
  •  Law of reflection    Incident, reflected ray and normal line lies on the same plane i=r Path of light reflection is represented in a ray diagram
  • Key terms in a ray diagrams       C – centre of curvature of mirrors or optical centre of lenses F – focus point of curved mirrors or lenses f – focal length (distance of F from C) u – distance of object to the surface of reflection or refraction v – distance of image to the surface of reflection or refraction Principle axis, P – light ray that is perpendicular to the surface of reflection or refraction and crosses through C and F * In concave and convex mirrors, C = 2F
  • Reflection on a plane mirror Object u Plane mirror v Image Line of sight
  • Properties of image formed    Same size as object v=u Laterally inverted    Flipped horizontally Left of Object becomes right of Image Virtual   Image exists within the mirror, a.k.a. another plane The image cannot be formed on a screen
  • Reflection on a concave mirror . . F C P
  • Reflection on a convex mirror P . . F C
  •       A concave mirror is also called a converging mirror A convex mirror is also called a diverging mirror The curvier the mirror, the smaller the focal length When incident rays are parallel to P, then reflected rays will pass through F When incident rays passes through F, then reflected rays will be parallel to P When incident rays passes through C, then reflected rays will pass through C in the opposite direction, parallel to the incident ray
  • Properties of image in concave and convex mirrors Location of object Properties of concave image u>C Real, Diminished, Inverted u=C Real, Same size as object, Inverted C<u<F Real, Magnified, Inverted u=F Formed at infinity u<F Virtual, Magnified, Upright Properties of convex image Virtual, Diminished, Upright
  • Applications of reflection       Parallax mirror in measuring instruments Wing and rear view mirrors Periscope Vanity mirror Satellite dish Headlights and torchlight reflectors
  • Refraction     Phenomenon where the speed of light changes as it propagates from one medium to another The change of speed causes a change in the direction of propagation When light propagates from a medium of low density to a medium of high density, its speed decreases, causing the direction of propagation to approach the normal The opposite is true when light passes from a medium of high density to a medium of low density
  •    Incident ray – i Refracted ray – r When light travels from a medium of low density to a medium of high density:     Its speed decreases Its direction changes i>r When light travels from a medium of high density to a medium of low density:    Its speed increases Its direction changes i<r
  • Low density High density r i r i>r Low density High density i i<r
  • Refractive Law   The incident ray, refracted ray and the normal line all lie on the same plane The ratio of sin i to sin r yields a constant known as the refractive index sin i = n, where n = refractive index sin r
  • Refractive Index, n      Has no units Indicates the light bending ability of a medium Value equals to the ratio of sin i to sin r Value also equals to ratio of speed of light in vacuum to speed of light in medium Value also equals to ratio of real depth to apparent depth
  • Total internal reflection      Is a form of light refraction. Occurs when light travels from a medium of high density to a medium of low density, where i > r. Occurs when the i is very large causing the r to be more than 90˚. Critical angle, c is the value of i that results in r = 90˚. When i > c, total internal reflection occurs and the reflected ray is present in the same medium as the incident ray.
  • Low density Low density r = 90˚ i=c Critical angle, c r > 90˚ High density i>c Total internal reflection High density
  • Observations and applications of refraction and total internal reflection         Sunset below the horizon Rainbow formation Mirages Fish’s eye view Fibre optics Prism periscope Prism binoculars Perfectly cut diamond
  • Lenses Bi Bi
  • Refraction through a biconvex lens F = Focal point C = Optical centre P = Principle axis f = focal length . . F C f . F f . P
  • Refraction through a biconcave lens F = Focal point C = Optical centre P = Principle axis f = focal length . . F C f . F f . P
  •        A concave lens is also called a diverging lens. A convex lens is also called a converging lens. The larger the lens, the larger the f value. The thicker the lens, the smaller the f value. When incident rays are parallel to P, then refracted rays will pass through F When incident rays passes through F, then refracted rays will be parallel to P No refraction occurs when incident rays passes through C. The rays simply pass through the lens in a straight line
  • Properties of image in convex and concave lenses Location of object Properties of convex image u=∞ Real, Diminished, Inverted u > 2f Real, Diminished, Inverted u = 2f Properties of concave image Real, Same size as object, Inverted Virtual, Diminished, Upright 2f < u < f Real, Magnified, Inverted u=f Image is formed at infinity u<f Virtual, Magnified, Upright
  •  Value of f    Value of u and v    Positive – convex lens Negative – concave lens Positive – real image Negative – virtual image Lens law     1/f = 1/u + 1/v f = focal length u = object length v = image length
  •  Linear magnification, m    m = v/u m = hi/ho, where hi = height of image and ho = height of object Power of lens, P   P = 1/f Unit = m-1 or Diopter (D)
  • Magnifying glass . . F C . F . P
  • Compound Microscope Construction line Eyepiece lens Objective lens Fo . Fo Fe . . Fe . Image is: •Magnified •Inverted •Virtual
  • Telescope Eyepiece lens Construction line Objective lens Fo . Fo / Fe . Fe . Image is formed at infinity