21. In which of the following materials does the light travel the fastest? (A) vacuum (B) water (n = 1.33) (C) glass (n = 1.50) (D) diamond (n = 2.42) (E) light travels at the same speed through all of the above materials

10. A copper telephone wire has essentially no sag between poles 35.0 m apart on a winter day when the temperature is -20.0°C. How much longer is the wire on a summer day when Tc = 35.0°C? Assume that the thermal coefficient of copper is constant throughout this range at its room temperature value. The average coefficient of linear expansion for copper is 17 × 10⁻⁶ (°C)⁻¹. (A) 3.27 cm (B) 4.12 cm (C) 1.00 cm (D) 12.04 cm (E) 8.40 cm

21. In which of the following materials does the light travel the fastest?
(A) vacuum
(B) water (n = 1.33)
(C) glass (n = 1.50)
(D) diamond (n = 2.42)
(E) light travels at the same speed through all of the above materials

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