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Inquiry: Plane Mirrors

Purpose: To verify the laws of reflection and to demonstrate image formation.


The laws of reflection state:

  • The incident ray, the reflected ray and the normal to the reflecting surface lie in the same plane.

  • The angle of incidence is equal to the angle of reflection.

  • The image of an object in a plane mirror is a virtual image. It is the same size as the object, erect, and equidistant behind the mirror as the object is in front.

Part A:

  1. Draw a line MN across the middle of a sheet of paper lengthwise.

  2. Place a mirror on line MN such that the back edge coincides with MN.

  3. Center a pin at a point about 4 cm from the mirror. This point is point P.

  4. Hold a ruler on edge such that angle AOD will be greater than 30 deg

  5. With one eyes sight along the ruler's edge the image of pin P.

  6. Draw a line along the ruler's edge. This line should be pointing to the image of the pin in the mirror.

  7. Repeat steps 4-6 for angle BCE.

  8. Remove mirror.

  9. All lines extending behind MN should be dashed. Extend the two sight lines until they intersect.(P1)

  10. Draw lines PO and PC.

  11. Draw the normal lines OD and CE.

  12. Measure distances HP and HP1

  13. Measure the angles of incidence POD and PCE.

  14. Measure the angles of reflection AOD and BCE.

Part B:

  1. Draw a line MN across the middle of a sheet of paper lengthwise.

  2. No less than 4 cm from the mirror draw a scalene triangle with sides from 4-7 cm

  3. Place a pin at each of the vertices of the triangle and label these pins A, B and C.

  4. Locate and draw two sight lines for the image of each pin.

  5. Label both sight lines corresponding to the point's label (A, B or C).

  6. Remove mirror.

  7. Extend the sight lines and label intersections corresponding to the objects label.

  8. Measure distance from the vertices to the mirror and the image points to the mirror.

  9. Measure and label the sides of the object triangle and the image triangle

Part C: Examining Corner Mirrors

Part C1:

  • Take the transparency with the word mirror on it and hold it in front of the mirror and examine.

  • Hold up your right hand, which hand does it look like is being held up.

Part C2:

  • Take a hinged mirror and set the angle at 180 degrees

  • Place an object about 4 cm in front of the mirror

  • Note the number of images.

  • Reduce the angle of the mirror and note the number of images again.

  • Note angles of back edges of mirror and note the number of images formed.

  • Validate that the below equation which relates the angle to the number of images is correct.

N = Int (360/A+1)

N=number of images formed
A = angle of mirror
Int = function that truncates behind the decimal
There are 360 deg in a circle