Chapter 5: Light and Vision CHAPTER 5: LIGHT AND VISION

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1 CHAPTER 5: LIGHT AND VISION These notes have been compiled in a way to make it easier or revision. The topics are not in order as per the syllabus. 5.1 Mirrors and Lenses Image Characteristics Image characteristics are described using the ollowing three categories: Size Same Image is exactly the same size as the object Magniied Image appears bigger than the object Diminished Image appears smaller than the object Direction Upright Image appears to be in the same direction as the object Inverted Image appears upside down compared to object Type Real Real images are images you can capture on a screen. Mirrors: Images are ormed on the same side o the mirror as the object Lenses: Images are ormed on the opposite side o the lens rom the object Virtual Virtual images are images you can see but cannot capture on a screen. Mirrors: Images are ormed on the opposite side o the mirror rom the object Lenses: Images are ormed on the same side o the lens as the object Plane mirrors i r Incident ray normal Relected ray Law o light relection: The relected angle is always the same as the incident angle. The incident ray, relected ray, and normal line are in the same plane. Characteristics o an image ormed by a plane mirror: Size Same Direction Upright, laterally inverted Type Virtual Distance Distance o an image rom the plane mirror is the same as the distance o the object rom the mirror Hoo Sze Yen Page 1 o 8

2 5.1.3 Curved Mirrors vs Lenses Concave mirror Convex mirror Also known as Converging mirrors Diverging mirror Focal lengths Positive E.g. = +20cm. Negative E.g. = -20cm. For both concave and convex mirrors, the ocal length is hal the radius; i.e. CF = FP. Convex lens Concave lens Also known as Converging lens Diverging lens Focal lengths Positive Negative E.g. = +20cm. E.g. = -20cm. Determining the Position and Characteristics o an Image with a Ray Diagram Concave mirror A ray parallel to the principal axis is relected to pass through F A ray through F is relected parallel to the principal axis Convex mirror A ray through C is relected back along its own path A ray parallel to the principal axis is relected as i it came rom F A ray towards F is relected parallel to the principal axis A ray towards C is relected back along its own path Hoo Sze Yen Page 2 o 8

3 Convex lens A ray parallel to the principal axis is reracted to pass through F A ray through F is reracted parallel to the principal axis Concave lens A ray through Ctravels straight along its own path A ray parallel to the principal axis is reracted as i it came rom F A ray towards F is reracted parallel to the principal axis A ray towards Ctravels straight along its own path To determine the position and characteristics o an image using a ray diagram: 1. Draw tworays emanating rom the top o the object to the mirror or lens, and using the guide in the table above, draw their relected/reracted paths. 2. The image is produced at the intersection o the two relected/reracted rays. Hoo Sze Yen Page 3 o 8

4 Position o object Between F and the mirror / lens Images ormed by a Concave Mirror / Convex Lens Ray diagram o concave mirrors Ray diagram o convex lenses Characteristics o image Virtual Upright Magniied At F Virtual Upright Magniied At ininity Between F and C/ 2F Real Inverted Magniied At C / 2F Real Inverted Same size Greater than C / 2F Real Inverted Diminished At ininity Real Inverted Diminished Position o object Anywhere in ront o the mirror or lens Images ormed by a Convex Mirror / Concave lens Ray diagram o convex mirror Ray diagram o concave lens Characteristics o image Virtual Upright Diminished Hoo Sze Yen Page 4 o 8

5 SUMMARY OF COMPARISON OF IMAGE CHARACTERISTICS Characteristics o concave mirrors are the same as convex lenses: Lens / Mirror 2 Diminished Real, Inverted Magniied Virtual, Upright Same size Object distance Image characteristics u = Real Inverted Diminished u >2 Real Inverted Diminished u = 2 Real Inverted Same Size < u <2 Real Inverted Magniied u = Virtual Upright Magniied u < Virtual Upright Magniied Characteristics o convex mirrors are the same as concave lenses: Virtual, Upright, Diminished Hoo Sze Yen Page 5 o 8

6 5.1.4 Lens Equation where u = object distance [cm] v = image distance [cm] = ocal length o lens [cm] Lens Power P 1 where P = lens power [D] = ocal length [m] OR 1 1 u v 1 P 100 where P = lens power [D] = ocal length [cm] Focal length, Convex lens: positive Concave lens: negative Object distance, u Always positive Image distance, v I positive: real image I negative: virtual image Linear Magniication Linear magniication is the ratio o the image size to the object size. where m = linear magniication h i = height o image h o = height o object m h h i o v u m > 1: magniied m = 1: same size m < 1: diminished I m is negative, take the modulus value Application o Lenses Complex Microscope o < e Astronomical Telescope o > e Magniication = e Normal setting: Length between lenses = o + e o Hoo Sze Yen Page 6 o 8

7 5.2 Reraction and Total Internal Relection Light reraction is a phenomenon where the direction o light is changed when it crosses the boundary between two materials o dierent optical densities. It occurs as a result o a change in the speed o light as it passes rom one medium to another. When a light ray travels rom medium A to medium B which is optically denser than A When a light ray travels rom medium C to medium D which is optically denser than C The ray o light will reract towards normal; r < i The ray o light will reract away rom normal; r > i When a light ray crosses the boundary between two dierent mediums at a right angle Snell s Law i = 0, r = 0 Snell s Law states that the ratio o sin i to sin r is a constant. sin i = constant sin r Reractive Index The reractive index or index o reraction o a medium is equivalent to the optical density o a medium. Note: A material with greater density may not necessarily have greater optical density. The reractive index / index o reraction o a medium, n can be calculated as: sin i n = sin r speedo light in air, c = speedo light in the medium, v actual depth, D = apparent depth, d 1 = sin c (where c is the critical angle) Hoo Sze Yen Page 7 o 8

8 5.2.3 Total Internal Relection Critical angle, c is the value o the incident angle when the reracted angle is 90. When iis increased to be greater than c, the light will be complete relected back into the material. No light will be reracted. This phenomenon is known as total internal relection. Conditions or total internal relection: 1. Light must be traveling rom an optically denser medium to a less dense medium. 2. The incident angle must be greater than the critical angle. END OF CHAPTER Hoo Sze Yen Page 8 o 8