Optics 2: REFRACTION & LENSES

REFRACTION Refraction: is the bending of waves because of the change of speed of a wave when it passes from one medium to another. The greater the difference in the speed of the waves, the greater the bending of the waves.

Normal: the line perpendicular to the surface. Incident ray: is the ray of light that strikes the surface. Reflected ray: is the ray of light that bounces off the surface. Refracted ray: is the ray of light that enters the material. Point of Incidence: is the point at which the light strikes the surface.

Angle of Incidence: the angle between the incident ray and the normal to the surface. Angle of Reflection: the angle between the reflected ray and the normal to the surface. Angle of Refraction: the angle between the refracted ray and the normal to the surface.

When a ray of light travels from a less dense medium to a denser medium; the refracted ray moves towards the normal to the surface. water Incident ray Refracted ray air

When a ray of light travels from a denser medium to a less dense medium; the refracted ray moves away from the normal to the surface. Refracted ray Incident ray air water

The amount of refraction depends upon: 1. The material through which the light travels. 2. The wavelength of light. The shorter the wavelength, the greater the refraction.

Violet light has a smaller wavelength than the other colors in the visible spectrum. Therefore, it is refracted the most.

THE LAW OF REFRACTION The greater the value of n, the greater the refraction.

MediumRefractive Index Vacuum1.00 Air Water1.33 Ethanol1.36 Crown glass1.52 Quartz1.54 Flint Glass1.62 Diamond2.42

Example #1 The index of refraction of water is Calculate the speed of light in water.

Example #2 Find the speed of light in a) ethanol b) quartz c) flint glass.

Example #3 The speed of light in plastic is 2 x 10 8 m/s. What is the refractive index of the plastic?

SNELL’S LAW

Example #4 A light ray of wavelength 589 nm traveling through air strikes a smooth, flat slab of crown glass (refractive index 1.52) at an angle of 30 0 to the normal. Find the angle of refraction. Incident ray Refracted ray Air=1.00 Glass=1.52

Example #5 Light in air is incident upon a piece of crown glass at an angle of What is the angle of refraction?

Example #6 A block of unknown material is submerged in water (refractive index is 1.33). Light in the water is incident on the block at an angle of The angle of refraction in the block is What is the index of refraction of the unknown material?

Incident ray Refracted ray Water=1.33 unknown

Critical Angle the incident angle that causes the incident ray to refract along the boundary between the two surfaces.  R = 90° Refracted ray Incident ray air water

When the angle of incidence is less than the critical angle; normal refraction takes place. Refracted ray Incident ray air water

When the angle of incidence is equal to the critical angle; the refracted ray is on the boundary. Refracted ray Incident ray air water

When the angle of incidence is greater than the critical angle; total internal reflection occurs. There is no refracted ray. Incident ray air water Reflected ray

TOTAL INTERNAL REFLECTION CONDITIONS Light is going from a denser medium to a less dense medium. Light is incident at an angle greater than the critical angle.

TOTAL INTERNAL REFLECTION OUTCOMES There is no refracted ray. There is only a reflected ray that stays in the denser medium. Total internal reflection and proper faceting causes diamonds and crystal glass to sparkle brilliantly.

EFFECTS OF REFRACTION Mirages Lingering daylight after the sun is below the horizon. An object in water, appears to be closer than it really is. Spoon appears bent when placed in a glass of water.

LENSES Lenses are curved transparent objects. They form images through refraction of light. They are usually made from plastic or glass. Two main types of lenses: 1.Convex 2. Concave

Convex Lens

C F C F

Focus or Focal Point: is the point on the principal axis where the parallel rays all come together after passing through the lens. The distance between the focus and the center of the lens is called the focal length. CC

RULES FOR DRAWING CONVEX LENS RAY DIAGRAMS Any ray parallel to the optical axis will be refracted through F. C F C F

RULES FOR DRAWING CONVEX LENS RAY DIAGRAMS Any ray passing through F will be refracted parallel to the principal axis. C F C F

RULES FOR DRAWING CONVEX LENS RAY DIAGRAMS Any ray passing through O continues straight through C F C F

When object is between F and O Image is Virtual Enlarged And Upright Used in a magnifying glass C F C F

When object is at F No image is formed Used in a telescope. C F C F

When object is between F and C Image is enlarged, real, and inverted Used in optical instruments. C F C F

When object is at C Image is same size, real, and inverted Used in optical instruments. C F C F

When object is beyond C Image is reduced, real, and inverted Used in optical instruments. C F C F

Concave Lens

C F C F

RULES FOR DRAWING CONCAVE LENS RAY DIAGRAMS Any ray parallel to the optical axis appears to be refracted through F. C F C F

RULES FOR DRAWING CONCAVE LENS RAY DIAGRAMS Any ray passing through O continues straight through C F C F

Object is placed between F and the Lens C F C F Image is reduced, virtual, and upright Used in telescopes and cameras.

Object is placed at F C F C F Image is reduced, virtual, and upright Used in telescopes and cameras.

Object is placed between F and C C F C F Image is reduced, virtual, and upright Used in telescopes and cameras.

Object is placed at C C F C F Image is reduced, virtual, and upright Used in telescopes and cameras.

Object is placed beyond C C F C F Image is reduced, virtual, and upright Used in telescopes and cameras.

Lens Equation f is negative if lens is concave d i is negative if image is virtual, if image is formed on the same side as the object d i is positive if image is real, if the image is formed on the opposite side as the object h i is negative if image is inverted M < 1 the image is smaller than the object M > 1 the image is larger than the object

Example #7 An object is 32 cm to the left of a convex lens of 8 cm focal length. Use the equation to locate the image.

Example #8 A 20 cm high object is placed 10 cm in front of a concave lens with a focal length of 10 cm. Find the image distance, magnification, and height. Describe the image. Draw a ray diagram.

C F C F

Example #9 An object is placed to the left of a 25 mm focal length convex lens, so that its image is the same size as the object. What is the image and object location?

Example #10 An object is placed 30 cm in front of a converging lens with a focal length of 10 cm. Find the image distance, magnification, and describe the image.

LASER Is a device that produces an intense, nearly parallel beam of coherent light. They are very high energy light sources.

EYE DEFECTS Farsightedness: person cannot focus near by objects. Far away objects appear clear while closer objects appear blurred. The person has an abnormality called hyperopia. The image is focused behind the retina.

CORRECTION OF FARSIGHTEDNESS

EYE DEFECTS Nearsightedness: person cannot focus far away objects. Near by objects appear clear while objects further away appear blurred. The person has an abnormality called myopia. The image is focused in front of the retina.

CORRECTION OF NEARSIGHTEDNESS