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When light from a point source is prevented from reaching a screen by a ‘ blocker’, a shadow that is the same shape as the blocker is cast on the screen. The position and relative size of the shadow can be determined by drawing in ‘boundary’ light rays that just pass the top and bottom of the blocker to reach the screen.

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We can represent light in diagrams by using a light ray model in which we draw light as straight arrows leading from one point to another. When light leaves a light source it travels in all directions (unless blocked), which can be represented by drawing several light rays emanating from the source in different directions. In order to see a light source, light from it must enter the observer’s eye.

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Any object we see is a certain color, because that object absorbs part of the light spectrum and reflects the rest. Objects follow the same rules as the gels (color subtraction). A yellow object absorbs the blue light, reflecting red and green into the observers eyes. Red + Green = Yellow. A red object absorbs blue and green, reflecting only red.

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When equal intensities of more than one of these color band enters the eye at the
same time we perceive different colors, by a process we call color addition:
R + G = Y (Yellow) R + B = M (Magenta)
B + G = C (Cyan) R + G + B = W (White)
Other colors can be created by varying the relative intensity of these three bands.

Minor Concept: Color gels (also called filters) work by absorbing certain color bands as light passes through them. Color inks and dyes also work in this way, which is called color subtraction.
R gel = –G, –B G gel = –R, –B B gel = –R, –G
Y gel = –B M gel = –G C gel = –R

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We usually identify several different colors in the visible spectrum. A common set is Red (R), Orange (O), Yellow (Y), Green (G), Blue (B), and Violet (V). However, when discussing color vision, for convenience, we divide the complete spectrum into three broad bands, Red (R), Green (G), and Blue (B). The three bands are similar to the way the light sensors in human eyes work. They come in 3 types: one sensitive to the red end of the spectrum, one sensitive to the middle part of the spectrum (green), and one sensitive to the blue end of the spectrum.

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Angles in less dense materials are BIGGER, Angles in more dense materials are SMALLER. NOTE: if light is in a dense material, it can get trapped, because for large angles of incidence in a dense material, refraction to the less dense material stops happening. 100% of the light reflects back into the dense material.

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When a light ray passes from one transparent material into another it changes
direction (eg air into glass, or water into air). This phenomenon is known as
refraction. There are approximate rules that help us draw light ray diagrams
involving refraction.

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When light reflects from a non-shiny surface (such as paper) each light ray reflects off the surface in multiple directions. This means that anyone looking at the surface will have light entering their eye and so be able to see it.

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When light reflects from a shiny surface (such as a mirror) each light ray obeys the law of reflection, which says that the angle of reflection (r), is always equal to the angle of incidence (i). These angles are always defined with respect to a ‘normal’ line (the dashed line passing through 0° and labeled ‘n’ in the diagram) that is drawn perpendicular to the surface at the point where the light ray strikes it.

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When light from a point source passes through a pinhole it continues in the same straight line and so produces a dot of light on a screen that is located at a point directly in line with the light ray passing through the hole. If more than one point source is used, each source will produce its own dot of light on the screen.