Light Optics
9th Grade Science · 6 lessons
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Lessons in this unit
- 1The Law of ReflectionLight reflects from a smooth surface with the angle of incidence equal to the angle of reflection, both measured from the normal. Rough surfaces obey the same law at every point, but their varying normals scatter the light — that is diffuse reflection.Experiment
- 2Refraction and Refractive IndexLight bends when it enters a medium where its speed differs. Going into a denser medium it slows and bends towards the normal. The refractive index is the ratio of speeds, and it is why a straw looks broken at the water line.Experiment
- 3Converging and Diverging LensesA convex lens brings parallel rays to a focus and can form a real, invertible image; a concave lens spreads them and always forms a virtual, upright, smaller one. Focal length sets the power: the shorter the focal length, the stronger the lens.Experiment
- 4Dispersion and ColourA prism separates white light because refractive index varies slightly with wavelength — violet slows most and bends most, red least. An object looks red because it reflects red wavelengths and absorbs the rest, not because it contains red light.Experiment
- 5The Eye and Correcting VisionThe cornea does most of the focusing and the lens fine-tunes it, projecting a real inverted image onto the retina. Short sight focuses in front of the retina and is corrected with a concave lens; long sight focuses behind it and needs a convex lens.Experiment
- 6Why the Sky Is BlueAir molecules scatter short wavelengths far more strongly than long ones. Blue light is therefore scattered across the whole sky while red passes through. At sunset the light travels further through the atmosphere, so even more blue is removed and the Sun looks red.Experiment
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