When light rays hit a smooth surface, it gets reflected. A plane mirror has a flat surface. What are the characteristics of an image formed by a plane mirror? imageobject REFLECTION IN A MIRROR

same size as the object distance of object from the mirror = distance of image from the mirror (a = b) ab CHARACTERISTICS OF IMAGE IN PLANE MIRROR

The image you see behind the mirror is due to the many light rays that bounce off you, onto the mirror, into your eyes. To locate the image, we draw ray diagrams. REFLECTION

O N A B incident ray reflected ray AO - incident ray Angle i - angle of incidence. NO, the perpendicular line to the surface, is also known as the normal. OB - reflected ray Angle r – angle of reflection. i r Angle of incidence = Angle of reflection RAY DIAGRAM (REFLECTION)

N incident ray A O reflected ray B The incident ray AO strikes the mirror at point O. Incident ray AO makes an angle of incidence, i, with the normal. The reflected ray OB travels into the boy’s eye. Ray OB makes an angle of reflection, r, with the normal. Angle i = angle r RAY DIAGRAM (REFLECTION) i r

1. Diverging light rays enter the eyes… HOW THE EYE SEES 2. Image is formed at back of eye ball 3. Brain processes the image

To locate the image… RAY DIAGRAM (REFLECTION) object O mirror

RAY DIAGRAM (REFLECTION) Measure equal distance away from mirror… object O image mirror If the image position is already given, you can skip this step.

Using only 1 ray from object to locate image… RAY DIAGRAM (REFLECTION) Note that the light rays are indicated using solid lines and arrows. All other lines are dashed. All arrows are directed towards the eye. object O image mirror

object O image RAY DIAGRAM (REFLECTION) If using 2 rays from object to locate image… (more common method) mirror Notice that the light rays do not converge at the eyes.

object O image mirror RAY DIAGRAM (REFLECTION) If using 2 rays from object to locate image… (more common method)

 Definition: refraction is the bending of light as light passes from one optical medium to another.  Similar to reflection, there is an angle of incidence ( i ) and an angle of refraction ( r )  (applet)  Can you use refraction to explain why white light forms a rainbow when it passes through a prism?

 Snell’s Law is very closely related to the concept of “refractive index”  Symbol for refractive index is n (no units)  Refractive Index is a measure of how fast (or how slow) light passes through a medium  There are 3 different ways to define refractive index:  First way: n = (speed of light in vacuum) / (speed of light in medium)  The first definition is also the “official” definition (in your syllabus)

 Hence, n for vacuum or air = 1  n for any other medium >1  Second way: n = (Sin i) / (Sin r)  IMPORTANT light MUST be coming from air (vacuum) and going into the medium (e.g. glass, water, etc.)  Recall Snell’s Law: (Sin i)/(Sin r) = constant  Can you see how they are related?  The third way will be discussed in the next sub-topic.

 Refractive Index (n) is a property of a material. The same material will always have the same n.  [Not in syllabus] MaterialRefractive Index (n) Vacuum1 (by definition) Air Water Plastic1.4 to 1.6 Glass1.5 to 1.9 Diamond2.419

 Magic Trick – Disappearing Coin   Disappearing Glass & Disappearing Gel 

 If the glass block has a refractive index of 1.5, what is angle x? (hint: need to use arc sine) 60° x

Step 1: Draw Normal! Step 2: i = 90 – 60 = 30° Step 3: Check: light going from air to medium? Ans: Yes! Step 4: Sin i / Sin r = n [IMPORTANT: presentation of working] Sin (30) / Sin r = 1.5 Sin r = Sin (30)/1.5 Sin r = (0.5)/1.5 = (4 sf) r = Sin -1 (0.3333) r = 19.5° (final answer?) No!! Qn ask for x not r!! r = 19.47° (2 dp) x = 90 – r = 90 – = 70.5° (1 dp) 60° x 30° r

 Always draw normal and measure i and r against normal!  Light entering at 90° doesn’t bend  Light bends towards the normal when it is traveling from lower to higher n  Light bends away from the normal when it traveling from higher to lower n  If you need to find n, but light is travelling from medium to air, use (sin r)/(sin i) instead of (sin i)/(sin r) [Qn: How come can we do this?]

 Sometimes the question uses the term “more dense” or “less dense”  This density refers to Optical Density, it is not to be confused with Physical Density (i.e. Mass / Volume)  More dense means higher n, less dense means lower n  When “less dense” think air  When “more dense” think water or glass

Note: the terms “real depth” and “apparent depth” are not in syllabus But you need to know that the image is more shallow than the object Eye Object Image

 [Not in syllabus]  Photographers: What is a fish eye lens?  Why is it called a “fish eye lens”?  Qn: There is a device inside your home (esp if you live in HDB flat) which is also a fish eye lens. What is it?  Ans: Doorviewer

[Not in syllabus] Source: idigitaldarwin.wordpress.com

 1) Draw a ray diagram of light going from glass to air, n = 1.5, i = 45°  What happened?  2) Find the angle of incidence when r = 90°  (What did you realize about sin 90?)  There is a special name for this angle, it is called the critical angle, or c

 Definition: the angle of incidence in an optically denser medium for which the angle of refraction in the optically less dense medium is 90°  Third definition of refractive index (n):  n = 1 / Sin c  Question: what happens when i > c?

 Important: Something which only occurs when light is going from more dense (i.e. higher n) to less dense (lower n)  TIR never occurs when light is going from less dense (lower n) to more dense (higher n)  Definition: The complete reflection of a light ray inside an optically denser medium at its boundary with an optically less dense medium  [applet demo]

 TIR is a case of reflection! That means angle of incidence = angle of reflection applies  When ray is going from higher n to lower n, always check for TIR!  Remember the two conditions for TIR:  From more dense medium to less dense medium  Angle of incidence > Critical Angle

 If the glass block has n = 1.5, complete the ray diagram below: 75°

n = sin i / sin r 1.5 = sin 75 / sin r r = 40.09° [What happens at the top surface?] [Check for TIR!] n = 1/sin c c = 41.8 [Is i>c? Yes] 75° 40.1° 49.9°

 Periscopes and Binoculars can be constructed either using mirrors (reflection) or using prisms (total internal reflection)  Prisms are preferred and is the better method.  Qn: Why?  Ans: There is double reflection in mirrors due to thickness of glass (remember your lab expt)  Important: Textbook Pg 234 has an error! Reward for someone who can spot and explain the error.

Source: sensorcentral.com

 (see textbook Pg 235)  Higher carrying capacity  Less signal degradation  Lightweight  Lower cost  Can be used as endoscopes for medical purposes

 McGyver overcomes the light sensors trap   The Engineer Guy: How Fiber Optic Cables work  I I

 3 definitions to memorize in this subtopic!  Refraction - is the bending of light as light passes from one optical medium to another.  Total Internal Reflection - The complete reflection of a light ray inside an optically denser medium at its boundary with an optically less dense medium  Critical Angle - the angle of incidence in an optically denser medium for which the angle of refraction in the optically less dense medium is 90°

 Snell’s Law: (Sin i)/(Sin r) = constant  3 equations for refractive index (n):  n = (speed of light in vacuum) / (speed of light in medium)  n = (Sin i) / (Sin r), provided light coming from air and going into the medium  n = 1 / Sin c, where c is critical angle

 Refraction as bending of light due to change in medium  Less dense to more dense – bend towards the normal  More dense to less dense – bend away from the normal  Use of Snell’s Law, Sine and Arc Sine to solve refraction problems  Critical Angle & Total Internal Reflection  Optical Fibres – how it works and its advantages