LIGHT OPTICS PART 1 REFLECTION Textbook Chapter 10

LIGHT / OPTICS – PART 1 (REFLECTION) Textbook: Chapter 10. 1 (pg 169 – 180)

Sub-topics 1. 2. 3. 4. Law of Reflection Single Ray Diagram (Straight surface) Single Ray Diagram (Curved surface) Multiple Ray Diagram (reflection of objects)

Image taken from grippod. com Perseus and the Gorgon (Medusa)

Recap from LSS � � � There is only one Physics concept in the subtopic of reflection. What is it? Ans: Angle of Incidence = Angle of Reflection Sometimes, this is called the “Law of Reflection” [not in syllabus]

How to draw a light ray � � Use ruler!!! Line must be straight – no breaks or kinks Draw an the “arrowhead” in the middle of the line, NOT at the end of the line MUST draw “arrowhead” to indicate direction

Single Ray reflection from a straight surface Reflected Ray Normal Incident Ray Angle of Incidence Reflecting Surface Angle of Reflection

Some tips � � � First thing you should always do is to draw the normal at the point where the incident ray touches the reflecting surface Use dashed lines to draw the normal Your drawing is only a sketch (i. e. an approximation). The angle of incidence only needs to look roughly equal to the angle of reflection (i. e. don’t waste time and use protractor to draw)

Single Ray reflection from a curved surface Reflected Ray Normal (to Tangent) Tangent Incident Ray Reflecting Surface

Some tips � When reflecting off a curved surface, always draw tangent first � Draw the tangent carefully! Don’t “intersect” the surface � � � Draw the normal perpendicular to the tangent. Don’t try to draw directly perpendicular to the surface – it is easy to get the normal off!! Make sure reflected angle looks roughly the same as incident angle

Common Misconception � Angle of Incidence and Angle of Reflection is always measured against the normal , it is never measured against the surface

Half-Time � The Wooden Mirror – can you explain how it works using the law of reflection? � http: //www. youtube. com/watch? gl=SG&hl=en- GB&v=BZysu 9 Qcce. M � Breaking the Magician’s Code – How to make an Elephant Disappear � http: //www. youtube. com/watch? v=JD_i 9 OY 9 g 4 k

Multiple Ray Reflection (Reflection of Objects) � � Only for straight surfaces (no curve surfaces) Recall from Biology (how the eye works) When we see an object, light travels from an object to the eye However, in order to see a an object in focus: 1. More than one light ray must diverge from the same point of the object 2. These light rays must re-converge back at a single point (usually using a lens) 3. This point must be captured on a screen (e. g. the back of the eyeball)

Diagram of eye Image taken from grade 10 optics. wikispaces. com

Multiple Ray Reflection � � 1. 2. In Physics, we will not be concerned about what goes on inside the eye, but only what happens outside the eye Similar principles apply: More than one ray must diverge from a single point Elsewhere, these rays (or their virtual rays) must re-converge back in a single point

Multiple Ray Reflection The point where the virtual rays meet is where the focused image is Single Point At least 2 diverging rays from single point If we repeat the process for other points, the whole object is reflected Draw normal for each ray Objectt For each ray, angle of reflection = angle of incidence These are called virtual rays. They should be straight lines “connected” to the reflected rays Reflecting Surface

Things to Note � � � Rays coming from point must be diverging! Use solid lines to draw real rays, but use dashed lines to draw virtual rays Each ray must obey the law of reflection (angle of incidence = angle of reflection) The virtual rays should connect to the reflected rays in a straight line You need to draw accurately, or else the image will appear off (e. g. the image does not appear to be the same height as the object)

Common Mistakes � Rays do not obey law of reflection upon hitting the reflecting surface � To � remind yourself, always draw a normal Reflected rays become parallel � Reflected � rays should also be diverging Virtual rays are not a straight connected line to the reflected rays � Try to draw angle of incidence = angle of reflection as accurately as possible

Reflected Image � � � Note that the reflected image is known as a “virtual image” This means that there are no real rays coming from the image. Virtual images are only formed using virtual rays. You will come across virtual images again in Light – Part 3.

Recap � � � Main concept: Angle of Incidence = Angle of Reflection Single Ray Reflection (straight surface) Single Ray Reflection (curve surface) � Draw � tangent first! Multiple Ray Reflection (reflecting objects) � Diverging rays from a single point � Each ray undergoes reflection � Virtual rays connected to reflected rays � Virtual rays meet to form image

Personal Card Feedback � � � If you think you are still unclear about single-ray reflections (either straight or curve surface), put a non-golden paperclip on your personal card If you think you are still unclear about multiple ray reflections, put a golden paperclip on your personal card If you think you understand both, do not put any paperclips.

Homework � � � Optic Worksheet 1 Amount of time required: around 1 hour Due: 1 week from today