Tham khảo tài liệu 'sat ii physics (sn) episode 2 part 5', ngoại ngữ, ngữ pháp tiếng anh phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | You can test all this yourself with the right kind of spoon. As you hold it at a distance from your face you see your reflection upside down. As you slowly bring it closer the upside-down reflection becomes blurred and a much larger reflection of yourself emerges this time right side up. The image changes from upside down to right side up as your face crosses the spoon s focal point. Convex Mirrors The focal point of a convex mirror is behind the mirror so light parallel to the principal axis is reflected away from the focal point. Similarly light moving toward the focal point is reflected parallel to the principal axis. The result is a virtual upright image between the mirror and the focal point. You ve experienced the virtual image projected by a convex mirror if you ve ever looked into a polished doorknob. Put your face close to the knob and the image is grotesquely enlarged but as you draw your face away the size of the image diminishes rapidly. The Two Equations for Mirrors and Lenses So far we ve talked about whether images are real or virtual upright or upside down. We ve also talked about images in terms of a focal length f distances d and and heights h and . There are two formulas that relate these variables to one another and that when used properly can tell whether an image is real or virtual upright or upside down without our having to draw any ray diagrams. These two formulas are all the math you ll need to know for problems dealing with mirrors and lenses. First Equation Focal Length The first equation relates focal length distance of an object and distance of an image 301 Values of d and f are positive if they are in front of the mirror and negative if they are behind the mirror. An object can t be reflected unless it s in front of a mirror so d will always be positive. However as we ve seen f is negative with convex mirrors and is negative with convex mirrors and with concave mirrors where the object is closer to the mirror than the focal point. A .
