Optics

Gabrielle DePetroAmy ChangTiffany Chau

Introduction to Optics

• Optics- study of how light behaves• Speed of light- 3 x 10^8 m/s• Speed of sound- 340 m/s• Light is both a wave and a particle– Wave: can undergo destructive interference,

refraction, diffraction– Particle: can travel through a vacuum

Reflection

• Reflection- the return of a wave back to its original medium

• Law of Reflection- states the angle of incidence and the angle of reflection are equal to each other

• Angle of Incidence- between the incident ray and the normal

• Angle of Reflection- between the reflected ray and the normal

• Normal- imaginary line perpendicular to a surface• Angle of Incidence = Angle of Reflection

Reflection and Mirrors

• Mirrors produce only virtual images• Virtual image- appears to be in location where

light does not really reach• Eye cannot tell difference between object and

virtual images• Image distance is same behind mirror as

object distance is in front of it• Image and Object are the same size

Refraction

• Refraction- bending of a wave as it crosses the boundary between two media at an angle

• Direction of light rays change because of refraction

• Causes of refraction:– Changes in speed of light crossing from one

medium to another– Variations in temperatures and densities of same

medium

Refraction and The Normal

• Depending on what medium they enter, light rays bend towards or away from the normal

• If light enters a medium in which it travels from:– Increasing to Decreasing speed, then rays bend

TOWARD the normal– Decreasing to Increasing speed, then rays bend

AWAY from the normal

Snell’s Law and Refraction

• Refraction- bending of light at the interface of 2 different materials

• Snell’s Law if the “equation” for refraction• Snell’s Law: n(in)(out)• n= index of refraction/property of the material• n (air)= 1.0• n (water)= 1.3

Critical Angle

• Critical angle- incident angle that causes the refracted ray to skim right along the boundary of a substance

• Produces an angle of reflection of 90°• Only exists when light is attempting to penetrate a

medium of higher optical density than it is currently traveling in

• If the angle of incidence exceeds the critical angle, the ray is completely reflected and does not enter the new medium

Total Internal Reflection

• Occurs when light attempts to pass from a more opticslly dense medium to a less dense medium at an angle greater than the critical angle

• There is no refraction, only reflection• Used for fiber optics

Lenses

• Lens- piece of glass or plastic that refracts light• Lenses change the path of light• A light ray bends as it enters and leaves glass• Lenses form an image by bending parallel rays of light

that pass through it• Light passing through certain shapes increases or

decreases image size and distance compared to the object being viewed

• Focal Length- distance between center of lens and its focal point

Converging Lenses

• Converging = Convex • Thicker in the middle• Causes rays that are initially parallel to meet at a single

point• Focal point- point at which beam of light parallel to the

principal axis converges• Any incident parallel beam converges to a point on the

focal plane• Lens has 2 focal points and 2 focal planes• Key features- principal axis, focal point, focal plane

Diverging Lenses

• Diverging = Concave• Thinner in the middle• Rays of light appear to originate from a single

point• Incident beam of light parallel to the principal

axis is diverged• Focal point- point at which light rays appear to

originate from

Ray Diagram Rules

• Ray diagrams show the principal rays that can be used to determine the size and location of an image

• An arrow represents the object and one end is on principal axis• To locate position of image, know the paths of 2 rays from a

point on the object• 3 Rules:

– Ray parallel to principal axis passes through the focal point on the opposite side

– Ray passing through the center of the lens that is undeflected– Ray through the focal point in front of the lens, emerges parallel to

principal axis after refraction

Image Formation

• Type of image formed depends on the shape of the lens and the position of the object

• Magnification- when the use of a lens allows an image to be observed through a wider angle than would be observed without the lens

• Magnifying Glass- converging lens that increases the angle of view and allows more detail to be seen

• Magnification Equation: -image size/object size

Images FormationFOR CONVERGING• Object is beyond focal point of converging lens:

– Real image, inverted– Slightly beyond focal point image is far away– Far from focal point image is nearer

• Object is between focal point and lens:– Virtual image, magnified, farther from the lens than the object, right-side up

FOR DIVERGING• No matter how far or near the object is: – Virtual, right-side up, smaller than the object

SUMMARY• For all Real Images, image and object are on opposite sides of the lens• For all Virtual Images, image and object are on same side of the lens

Multiple Choice Review

1. Convex lenses are…A)ConvergentB)DivergentC)MirrorsD)Polarized 2. The image formed my concave lenses is always….E)Real and InvertedF)Real and Upright G)Virtual and InvertedH)Virtual and Upright

3. For an object with dₒ = 12m and focal length = -4m, what is the distance of the image? A) 0.1m B) -3mC) -30m D) 10m4. ________ is an optical device that works by reflection and _______ is an optical device that works by refraction.E) Mirror, lensF) Lens, mirrorG) Mirror, mirrorH) Lens, lens

5. A concave lens is _______ and _______.A)Convergent, magnifiesB)Convergent, reducesC)Divergent, magnifiesD)Divergent, reduces6. Nearsightedness can be corrected by…E)Convex lensesF)Convex mirrors G)Concave lensesH)None of the above

7. Calculate the critical angle for light going from water into airA) 4.87°B) 48.75°C) 487.53°D) 90°8. (In the image below) As the image is moved from the red arrow to the green arrow it…

E) is closer and increases in sizeF) is closer and decreases in sizeG) is farther and increases in sizeH) is farther and decreases in size

9. If an object is 8m from a lens and produces an image 4m from the lens, what is the magnification factor, M?A) -0.5B) 0.5C) -0.05D) 0.0510. The angle of reflection is 30°. What is the angle of incidence?E) 30°F) 60°G) 90°H) 150°•  

Answers

• ANSWERS: 1) A 2) D 3) B 4) A 5) D 6) C 7) B 8) C 9) A 10) A