Light and Reflection Level 1 Physics

Facts about Light It is a form of Electromagnetic Energy It is a part of the Electromagnetic Spectrum and the only part we can really see

More facts… Speed of light, c, in a vacuum is a constant value Light can be: Reflected Refracted Absorbed Light is an electromagnetic wave where electric and magnetic fields fluctuate together.

Reflection of Light Angle of Incidence θ i Angle of Reflection θ r Incident Ray Reflected Ray Mirror Law of Reflection: The angle of incidence (θ i ) EQUALS the angle of reflection (θ r ) Both angles are measured from a line drawn perpendicular to the surface: NORMAL

SPECULAR VS. DIFFUSE Specular: Important in determining properties of mirrors. Rays are always parallel to one another. Diffuse: Law of reflection applies to each ray, but irregular surface reflects light in various directions. Size of irregularity is ≥ the wavelength of light

Plane Mirror A duck is located 15 cm in front of a vertical mirror. WHERE IS THE OF THE DUCK LOCATED? MIRROR Object Distance, D o = 15 cm On the same side? Behind the mirror? On the surface?

Plane Mirror A duck is located 15 cm in front of a vertical mirror. WHERE IS THE OF THE DUCK LOCATED? MIRROR Object Distance, D o = 15 cm Behind the mirror? Virtual Image Image Distance, D i = 15 cm Object height h o Image height h i

Virtual Image To the eye, it appears that the ray originates from behind the mirror Rays of light seem to Come from the image, But they DO NOT originate From behind the mirror Since none of the rays actually come from the image it is called a VIRTUAL IMAGE

Virtual Images continued… Virtual Images are basically images which cannot be visually projected on a screen. If this box gave off light, we could project an image of this box on to a screen provided the screen was on the SAME SIDE as the box. You would not be able to project the image of the vase or your face in a mirror on a screen, therefore it is a virtual image. CONCLUSION: VIRTUAL IMAGES are ALWAYS on the OPPOSITE side of the mirror relative to the object.

Real Image If the image can be seen on a screen, it is considered a REAL IMAGE Real images are always on SAME SIDE of the mirror as the object (for MIRRORS) The characteristics may be different from the object: SIZE – reduced, same, enlarged POSITION - same side or opposite side ORIENTATION – right side up or inverted

Spherical Mirrors Also called DIVERGING mirrorAlso called CONVERGING mirror

Concave Mirror - Converging A converging mirror will focus parallel light rays to a single (fixed) point in front of the surface of the mirror. This point is said to be the FOCAL POINT. To find this point, you must use light from INFINITY

Converging Mirror Thought to be a slice of a sphere and having a line drawn through The exact center of the mirror – Principle Axis. Since it is technically a sphere, the mirror has a CENTER OF CURVATURE, C The focal length, f is half the distance of C

Ray Diagrams Principal Ray Pineault: Draw a ray from the top of the object parallel to the principal axis and then through f after reflection. fC Principal Axis

Ray Diagrams Vice Principal Ray Lanczycki: Draw a ray from the top of the object through the focal point, then parallel to the principal axis. fC Principal Axis

Ray Diagrams Vice Principal Ray Alves: Draw a ray from the top of the object, through C, then back along itself. fC Principal Axis

Ray Diagrams fC Principal Axis

Converging Mirror – Image Characteristics fC Principal Axis Is the image on the SAME or OPPOSITE side of mirror as object? SAME SIDE, therefore considered a REAL IMAGE Is the image ENLARGED or REDUCED? Is the image INVERTED or UPRIGHT?

Diverging Mirror Reflected rays will NEVER cross on object side of mirror, therefore, continue reflected rays behind the mirror to locate the image. Image is: REAL or VIRTUAL SMALLER or BIGGER INVERTED OR UPRIGHT

Diverging Mirror A diverging mirror (convex) will ALWAYS form a VIRTUAL IMAGE Because of its shape, a diverging mirror gives a wider field of view than other mirrors.

The Mirror/Lens Equation Is there any OTHER way to predict image characteristics besides the ray diagram? YES! One way is to use the MIRROR/LENS equation to CALCULATE the position of the image.

Mirror/Lens Equation Assume that a certain concave spherical mirror has a focal length of 10.0 cm. Locate the image for an object distance of 25 cm and describe the image’s characteristics cm What does this tell us? First we know the image is BETWEEN “C” & “f”. Since the image distance is POSITIVE the image is a REAL IMAGE. Real image = positive image distance Virtual image = negative image distance What about the size and orientation?

Magnification Equation To calculate the orientation and size of the image we use the MAGNIFICATION EQUATION. Here is how this works: If we get a POSITIVE magnification, the image is UPRIGHT. If we get a NEGATIVE magnification, the image is INVERTED If the magnification value is GREATER than 1, the image is ENLARGED. If the magnification value is LESS than 1, the image is REDUCED. If the magnification value is EQUAL to 1, the image is the SAME SIZE as the object. Using our previous data we see that our image was INVERTED, and REDUCED.

Example Assume that a certain concave spherical mirror has a focal length of 10.0 cm. Locate the image for an object distance of 5 cm and describe the image’s characteristics. -10 cm 2x VIRTUAL (opposite side) Enlarged Upright Characteristics?